PCI/pwrctrl: Add PCIe M.2 connector support

PCI/pwrctrl: Add PCIe M.2 connector support

Add support for handling PCIe M.2 connectors as Power Sequencing devices.
These connectors are exposed as Power Sequencing devices as they often
support multiple interfaces like PCIe/SATA, USB/UART to the host machine,
and the interfaces may be driven by different client drivers at the same
time.

This driver handles the PCIe interface of these connectors. It first checks
for the presence of the graph port in the Root Port node with the help of
of_graph_is_present() API. If present, it acquires/powers ON the
corresponding pwrseq device.

Once the pwrseq device is powered ON, the driver will skip parsing the Root
Port/Slot resources and register with the pwrctrl framework.

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@oss.qualcomm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Link: https://patch.msgid.link/20260128-pci-m2-v7-1-9b3a5fe3d244@oss.qualcomm.com