powerpc/85xx: Workaround MPC8572/MPC8536 GPIO 1 errata.

On MPC8572 and MPC8536 the status of GPIO pins configured
as output cannot be determined by reading GPDAT register.
Workaround by reading the status of input pins from GPDAT
and the status of output pins from a shadow register.

Signed-off-by: Felix Radensky <felix@embedded-sol.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>