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>

drivers/pinctrl/pinctrl-u300.c

@@ -670,7 +670,7 @@ struct u300_pmx {
  * u300_pmx_registers - the array of registers read/written for each pinmux
  * shunt setting
  */
-const u32 u300_pmx_registers[] = {
+static const u32 u300_pmx_registers[] = {
 	U300_SYSCON_PMC1LR,
 	U300_SYSCON_PMC1HR,
 	U300_SYSCON_PMC2R,