pinctrl: spawn U300 pinctrl from the COH901 GPIO

This solves the riddle on how the U300 pin controller shall be
able to reference the struct gpio_chip even though these are
two separate drivers: spawn the pinctrl child from the GPIO
driver and pass in the struct gpio_chip as platform data.
In the process we rename the U300 "pinmux-u300" to
"pinctrl-u300" so as not to confuse.

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