phy: sun4i-usb: add support for missing USB PHY index

The new Allwinner H6 SoC's USB2 PHY has two holes -- USB1 (which is a
3.0 port with dedicated PHY) and USB2 (which doesn't exist at all).

Add support for this kind of missing USB PHY index.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>