arm64: dts: mediatek: mt7981b: Add PCIe and USB support

arm64: dts: mediatek: mt7981b: Add PCIe and USB support

Add device tree nodes for PCIe controller and USB3 XHCI host
controller on MT7981B SoC. Both controllers share the USB3 PHY
which can be configured for either USB3 or PCIe operation.

The USB3 XHCI controller supports USB 2.0 and USB 3.0 SuperSpeed
operation. The PCIe controller is compatible with PCIe Gen2
specifications.

Also add the topmisc syscon node required for USB/PCIe PHY
multiplexing.

Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Sjoerd Simons <sjoerd@collabora.com>
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>