ARM: sun6i: a31s: primo81: Change USB OTG to OTG mode

Now that we have support for the AXP221 PMIC's USB VBUS detection and
DRIVEVBUS vbus control, we can use the USB OTG port in proper OTG mode.

This patch enables the aforementioned PMIC functions, adds the OTG ID
detection pin to the USB PHY node, and changes the mode of USB OTG to
"otg".

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>