sunxi: phy: Add USB PHY support for Allwinner H6

The USB PHY used in the Allwinner H6 SoC has some pecularities (as usual),
which require a small addition to the USB PHY driver:
In this case the second PHY is PHY3, not PHY1, so we need to skip number
1 and 2 in the code. Just use the respective code from Linux for that.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Tested-by: Corentin Labbe <clabbe.montjoie@gmail.com> # Pine-H64
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>