gpio: mpc8xxx: don't do RMW on gpdat register when setting value

The driver correctly handles reading back the value of an output gpio
by reading from the shadow register for output, and from gpdat for
inputs.

Unfortunately, when setting the value of some gpio, we do a RMW cycle
on the gpdat register without taking the shadow register into account,
thus accidentally setting other output gpios (at least those whose
value cannot be read back) to 0 at the same time.

When changing a gpio from input to output, we still need to make sure
it initially has the requested value. So, the procedure is

- update the shadow register
- compute the new gpdir register
- write the bitwise and of the shadow and new gpdir register to gpdat
- write the new gpdir register

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>