gpio: mpc8xxx: don't allow input-only pins to be output for MPC5121

Add a 5121-custom reject if an input-only pin is requested to be output
(see 18.3.1.1 in the refman). Also, rewrite mach-specific quirk setup to
consume less lines which scales better.

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
[grant.likely: Fixed build error]
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>