dt-bindings: PCI: mediatek-gen3: Add MT7981 PCIe compatible

dt-bindings: PCI: mediatek-gen3: Add MT7981 PCIe compatible

Add compatible string for MediaTek MT7981 PCIe Gen3 controller.
The MT7981 PCIe controller is compatible with the MT8192 PCIe
controller.

Signed-off-by: Sjoerd Simons <sjoerd@collabora.com>
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://patch.msgid.link/20251223-openwrt-one-network-v5-1-7d1864ea3ad5@collabora.com