Set Rev 2.x 86xx PIC in mixed mode.

Prevent false interrupt from hanging Linux as MSR[EE] is set
to enable interrupts by changing the PIC out of the default
pass through mode into mixed mode.

Signed-off-by: Haiying Wang <haiying.wang@freescale.com>
Signed-off-by: Jon Loeliger <jdl@freescale.com>