Backport: PCIe M.2 connector support and WCN7850 BT integration

[WeiDeng-weiden]

This PR backports the PCIe M.2 connector framework from qcom-next to qcom-6.18.y,
so that M.2-connected WCN7850 wireless cards can be properly enumerated and powered through
devicetree.

The framework consists of: DT bindings for PCIe M.2 Mechanical Key M and Key E connectors; the
power-sequencing driver pwrseq-pcie-m2 which handles regulators, GPIO control of W_DISABLE1# /
W_DISABLE2#, and powerup delays; PCI/pwrctrl integration that creates a pwrctrl device when a
graph port is present in DT; and serdev API extensions allowing dynamic creation of UART child
devices under an external connector via OF graph traversal.

The last three commits in the list are pwrseq-pcie-m2 fixes that landed shortly after the main
series; they are cherry-picked together to keep the driver consistent with mainline.

Commits (qcom-6.18.y<- qcom-next):

c5fa5a8 <- 926194a dt-bindings: connector: Add PCIe M.2 Mechanical Key M connector
ad4c382 <- 52e7b5b power: sequencing: Add the Power Sequencing driver for the PCIe
M.2 connectors
52a4471 <- 9db8262 PCI/pwrctrl: Create pwrctrl device if graph port is found
e63ff5d <- 9cb64f6 PCI/pwrctrl: Add PCIe M.2 connector support
5fbee2d <- e7fef85 serdev: Convert to_serdev_*() helpers to macros and use
container_of_const()
8fbc83c <- a2b4814 serdev: Add an API to find the serdev controller associated with
the devicetree node
76729ef <- 92fa16e serdev: Do not return -ENODEV from of_serdev_register_devices() if
external connector is used
23be572 <- 1785c7b dt-bindings: serial: Document the graph port
1825e60 <- 5970c1d dt-bindings: connector: Add PCIe M.2 Mechanical Key E connector
5e157e9 <- 0d38285 power: sequencing: pcie-m2: Add support for PCIe M.2 Key E
connectors
2e4a948 <- 3f736ae power: sequencing: pcie-m2: Create serdev device for WCN7850
bluetooth

Fixes (also cherry-picked from mainline):

699295c <- 19b8c8f power: sequencing: pcie-m2: enforce PCI and OF dependencies
19b2381 <- b4464d8 power: sequencing: pcie-m2: add SERIAL_DEV_BUS dependency
8a512e4 <- 5c3daa5 power: sequencing: pcie-m2: Fix device node reference leak in
probe

Compatibility modifications:

1. kzalloc_obj() -> kzalloc()
   Mainline commit 3f736ae uses kzalloc_obj(), introduced in v7.0 by 2932ba8 ("slab:
   Introduce kmalloc_obj() and family"); not available in qcom-6.18.y. Local commit 2e4a948
   replaces ctx->ocs = kzalloc_obj(*ctx->ocs); with the equivalent ctx->ocs =
   kzalloc(sizeof(*ctx->ocs), GFP_KERNEL);. Behaviour is identical; only the call form changes.

2. PCI_PWRCTRL_GENERIC vs PCI_PWRCTRL_SLOT naming
   Mainline commit 9cb64f6 modifies drivers/pci/pwrctrl/slot.c and the PCI_PWRCTRL_SLOT
   Kconfig symbol. qcom-6.18.y carries an existing FROMLIST commit (fc4a55d) that
   reverse-renamed slot.c -> generic.c and PCI_PWRCTRL_SLOT -> PCI_PWRCTRL_GENERIC. Local commit
   e63ff5d keeps qcom-6.18.y's naming while applying mainline's skip_resources: refactoring
   for the pwrseq path.

CRs-Fixed: 4550026