dt-bindings: serial: Document the graph port

dt-bindings: serial: Document the graph port

A serial controller could be connected to an external connector like PCIe
M.2 for controlling the serial interface of the card. Hence, document the
OF graph port.

Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Reviewed-by: Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@oss.qualcomm.com>
Link: https://patch.msgid.link/20260326-pci-m2-e-v7-4-43324a7866e6@oss.qualcomm.com
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>