phy: amlogic: phy-meson-gxl-usb2: support the clock and reset line

The Meson GXL USB2 PHYs require an additional clock (USB) which has to
be enabled. If that clock is disabled then all PHY registers read 0x0.
Luckily for us that clock is always enabled (either by harddware
defaults, the bootrom, or any of the bootloaders before u-boot/BL3-3).

The OTG capable USB2 PHY additionally has a reset line (USB_OTG, which
is shared with other components, such as the USB3 PHY for example).

Extend the driver so it handles this clock and the shared reset line.
We only trigger the reset during the .init phase since it's a shared
reset line, so triggering it during the driver's .reset implementation
would effectively also only trigger it once anyways.

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