phy: allwinner: add phy driver for USB3 PHY on Allwinner H6 SoC

Allwinner H6 SoC contains a USB3 PHY (with USB2 DP/DM lines also
controlled).

Add a driver for it.

The register operations in this driver is mainly extracted from the BSP
USB3 driver.

Signed-off-by: Ondrej Jirman <megous@megous.com>
Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>