gpio: mpc8xxx: Preserve pre-init state of outputs

The mpc8xxx_gpio driver contains a workaround for certain chips
where the previously written state of outputs cannot be read back
from the GPIO data (GPDAT) register (MPC8572/MPC8536). This workaround
consists of tracking the state of GPDAT in a "shadow register" (i.e. a
software variable). The shadow register is initialized to zero.

This results in a problem w.r.t. outputs that are configured to a
high (1) state before U-Boot runs, but not touched by U-Boot itself:
Due to the zero-initialization, these GPIOs end up being set to zero,
the first time that any other output is set.

To avoid such issues initialize the GPDAT shadow register to the value
previously held by any outputs, if possible. On MPC8572/MPC8536 this
should make no difference, i.e. the shadow register should be
initialized to zero on these chips.

This patch has been tested on a MPC8314E-based board.

Reviewed-by: Sinan Akman <sinan@writeme.com>
Signed-off-by: J. Neuschäfer <j.ne@posteo.net>

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