MUSB_HDRC: Allow selecting OTG, peripheral or host mode via sysfs

MUSB_HDRC: Allow selecting OTG, peripheral or host mode via sysfs

This can be used on systems that don't use ID pin, or have mini-B
connector.

For example, to force N800 into host mode with non-standard mini-B
to mini-B cable connected to a powered hub:

Note that connected mini-A cable cannot be forced to peripheral
mode as the ID pin is grounded by the cable. Also note that
any VBUS load above the 100mA will cause the host mode to fail.

Signed-off-by: Tony Lindgren <tony@atomide.com>