pinctrl: u300: make u300_pmx_registers static

The array u300_pmx_registers is not declared or used outside
of the driver, so make it static to avoid the following warning:

drivers/pinctrl/pinctrl-u300.c:673:11: warning: symbol 'u300_pmx_registers' was not declared. Should it be static?

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>