powerpc/85xx: fix mpic configuration in CAMP mode

Change the string to check for CAMP mode boot on MPC85xx (eg. P2020) to match
the one in the corresponding dts files (p2020rdb_camp_core{0,1}.dts).

Without this fix the mpic is configured as in the SMP boot mode, which causes
the first core to report a protected source interrupt error for devices
of the other core and lock up.

Also add MPIC_SINGLE_DEST_CPU on both P2020 based architectures in CAMP
mode as suggested by Scott Wood. Thanks.

Cc: Scott Wood <scottwood@freescale.com>
Cc: Poonam Aggrwal <poonam.aggrwal@freescale.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Fabio Baltieri <fabio.baltieri@gmail.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>