phy: meson-g12a-usb3-pcie: Add support for PCIe mode

This adds extended PCIe PHY functions for the Amlogic G12A
USB3+PCIE Combo PHY to support reset, power_on and power_off for
PCIe exclusively.

With these callbacks, we can handle all the needed operations of the
Amlogic PCIe controller driver.

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>