arm64: dts: meson-g12a: Add PCIe node

This adds the Amlogic G12A PCI Express controller node, also
using the USB3+PCIe Combo PHY.

The PHY mode selection is static, thus the USB3+PCIe Combo PHY
phandle would need to be removed from the USB control node if the
shared differential lines are used for PCIe instead of USB3.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Andrew Murray <andrew.murray@arm.com>