powerpc/5200: Refactor mpc5200 interrupt controller driver

Rework the mpc5200-pic driver to simplify it and fix up the setting
of desc->status when set_type is called for internal IRQs (so they
are reported as level, not edge).  The simplification is due to
splitting off the handling of external IRQs into a separate block
so they don't need to be handled as exceptions in the normal
CRIT, MAIN and PERP paths.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>