mpc8[5/6]xx: Ensure POST word does not get reset

The POST word is stored in a spare register in the PIC on MPC8[5/6]xx
processors.  When interrupt_init() is called, this register gets reset
which resulted in all POST_RAM POSTs not being ran due to the corrupted
POST word.  To resolve this, store off POST word before the PIC is
reset, and restore it after the PIC has been initialized.

Signed-off-by: John Schmoller <jschmoller@xes-inc.com>
Signed-off-by: Peter Tyser <ptyser@xes-inc.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>