85xx: Improve MPIC initialization

The MPIC initialization code for Freescale e500 CPUs was not using I/O
accessors, and it was not issuing a read-back to the MPIC after setting
mixed mode.  This may be the cause of a spurious interrupt on some systems.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>