pinctrl: nomadik: add USB functions for STn8815

The MUSB block in the Nomadik has two pin settings: high speed or
full speed. These correspond to two unique pin group settings: all
pins set to function B for high speed and all set to function C
for full speed. Full speed uses more pins than high speed.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>