dt-bindings: connector: Add PCIe M.2 Mechanical Key M connector

dt-bindings: connector: Add PCIe M.2 Mechanical Key M connector

Add the devicetree binding for PCIe M.2 Mechanical Key M connector defined
in the PCI Express M.2 Specification, r4.0, sec 5.3. This connector
provides interfaces like PCIe and SATA to attach the Solid State Drives
(SSDs) to the host machine along with additional interfaces like USB, and
SMBus for debugging and supplementary features.

The connector provides a primary power supply of 3.3v, along with an
optional 1.8v VIO supply for the Adapter I/O buffer circuitry operating at
1.8v sideband signaling.

The connector also supplies optional signals in the form of GPIOs for fine
grained power management.

Reviewed-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@oss.qualcomm.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://lore.kernel.org/r/20260122-pci-m2-v6-1-575da9f97239@oss.qualcomm.com
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>