gpio: mpc8xxx: latch GPIOs state on module load when configured as output

Qoriq and related devices allow reading out state of GPIO set as output.
However, currently on driver's init, all outputs are configured as driven
low. So, any changes to GPIO confiuration will drive all pins (configured
as output) as output-low.
This patch latches state of output GPIOs before any GPIO configuration
takes place. This preserves any output settings done prior to loading
the driver (for example, by u-boot).

Signed-off-by: Michal Smulski <michal.smulski@ooma.com>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>