dt-bindings: PCI: mediatek: Convert to YAML schema

dt-bindings: PCI: mediatek: Convert to YAML schema

Convert the PCI mediatek Documentation to YAML schema to enable
validation of the supported GEN1/2 Mediatek PCIe controller.

While converting, lots of cleanup were done from the .txt with better
specifying what is supported by the various PCIe controller variant and
drop of redundant info that are part of the standard PCIe Host Bridge
schema.

To reduce schema complexity the .txt is split in 2 YAML, one for
mt7623/mt2701 and the other for every other compatible.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://patch.msgid.link/20251020111121.31779-2-ansuelsmth@gmail.com