phy: amlogic: Add Amlogic G12A USB3 + PCIE Combo PHY Driver

This adds support for the shared USB3 + PCIE PHY found in the
Amlogic G12A SoC Family.

It supports USB3 Host mode or PCIE 2.0 mode, depending on the layout of
the board.

Selection is done by the #phy-cells, making the mode static and exclusive.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>