powerpc/mpc5200: Make internal 5200 PIC the default interrupt controller

The MPC5200 internal interrupt controller setup function needs to set
the default interrupt controller when it is called.  Without this
irq_create_of_mapping() cannot be called without first determining
the pointer to the irq controller (ie. call with controller = NULL).

Reported-by: Steven Cavanagh <scavanagh@secretlab.ca>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>