ARM: sun8i: h2+: enable USB OTG for Orange Pi Zero board

Orange Pi Zero board features a USB OTG port, which has a ID pin, and
can be used to power up the board. However, even if the board is powered
via +5V pin in GPIO/expansion headers, the VBUS in the OTG port cannot
be powered up, thus it's impossible to use it in host mode with simple
OTG cables.

Add support for it in peripheral mode.

If someone really want to use it in host mode, the mode of PHY can be
switch via sysfs, then use a powered USB OTG cable or powered USB HUB to
power up external USB devices.

Signed-off-by: Icenowy Zheng <icenowy@aosc.xyz>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>