powerpc/5xxx: extend mpc8xxx_gpio driver to support mpc512x gpios

The GPIO controller of MPC512x is slightly different from
8xxx GPIO controllers. The register interface is the same
except the external interrupt control register. The MPC512x
GPIO controller differentiates between four interrupt event
types and therefore provides two interrupt control registers,
GPICR1 and GPICR2. GPIO[0:15] interrupt event types are
configured in GPICR1 register, GPIO[16:31] - in GPICR2 register.

This patch adds MPC512x speciffic set_type() callback and
updates config file and comments. Additionally the gpio chip
registration function is changed to use for_each_matching_node()
preventing multiple registration if a node claimes compatibility
with another gpio controller type.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>