Add ux500_musb_set_vbus() implementation for ux500.

This is based on the version originally developed inside ST-Ericsson.
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Fabio Baltieri <fabio.baltieri@linaro.org>

[ balbi@ti.com: fix a build error due to
	missing otg_state_string() ]
Signed-off-by: Felipe Balbi <balbi@ti.com>