pinctrl: rockchip: make rockchip_irq_gc_mask_set_bit static

The rockchip_irq_gc_mask_set_bit() function is not exported our used
outside of ppinctrl-rockchip.c so fix the following sparse error by
making it static:

drivers/pinctrl/pinctrl-rockchip.c:2010:6: warning:
  symbol 'rockchip_irq_gc_mask_set_bit' 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>