On rk3288, for gpio bank 0, the registers which configure pull-up,
iomux, and drive strength don't implement the enable bits in the upper
half of the register, unlike the other gpio configuration registers,
and so the kernel must perform a read-modify-write of the register to
update a particular gpio in that bank.

The current code is actually clobbering the contents of the register,
so this fixes it by using regmap_update_bits and masking out only the
bits which require updating.  In the case of bank0 on rk3288 the upper
enable bits will just get ignored, and the other configurations won't
get clobbered.
Signed-off-by: Sonny Rao <sonnyrao@chromium.org>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>