Add support for the Amlogic G12A SoC using a separate shared PHY.

This adds support for fetching a PHY phandle and call the PHY init,
reset and power on/off calls instead of writing in the PHY register or
toggling the PHY reset line.

The MIPI clock and the PHY memory resource are only required for the
Amlogic AXG SoC PCIe PHY setup, thus these elements are ignored for
the Amlogic G12A having a separate shared PHY.
Signed-off-by: NNeil Armstrong <narmstrong@baylibre.com>
Signed-off-by: NLorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: NAndrew Murray <andrew.murray@arm.com>