Add the Amlogic G12A Family USB3 + PCIE Combo PHY Bindings.

This PHY can provide exclusively USB3 or PCIE support on shared I/Os.
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>