rockchip: rk3288: Fix pinctrl for GPIO bank 0

rockchip: rk3288: Fix pinctrl for GPIO bank 0

Bank 0 is the "PMU GPIO" bank which is controlled by the PMU registers
rather than the GRF registers.  In the GRF the top half of the register
is used as a mask so that some bits can be updated without affecting the
others, but in the PMU this feature is not provided and the top half of
the register is reserved.

Take the same approach as the Linux driver to update the value via
read-modify-write but setting the mask for only the bits that have
changed.  The PMU registers ignore the top 16 bits so this works for
both GRF and PMU iomux registers.

Signed-off-by: John Keeping <john@metanate.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>