[POWERPC] mpc5200: add interrupt type function

Add a set_type function for external (GPIO) interrupts.

Signed-off-by: Juergen Beisert <j.beisert@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>