phy: meson: add USB2 PHY support for Meson GXL and GXM

phy: meson: add USB2 PHY support for Meson GXL and GXM

This adds a new driver for the USB2 PHYs found on Meson GXL and GXM SoCs
(both SoCs are using the same USB PHY register layout).

The USB2 PHY is a simple PHY which only has a few registers to configure
the mode (host/device) and a reset register (to enable/disable the PHY).

Unfortunately there are no datasheets available for this PHY. The driver
was written by reading the code from Amlogic's GPL kernel sources and
by analyzing the registers on an actual GXL and GXM device running the
kernel that was shipped on the boards I have.

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