ppc/85xx,86xx: Handling Unknown SOC version

Incase the system is detected with Unknown SVR, let the system boot
with a default value and a proper message.

Now with dynamic detection of SOC properties from SVR, this is necessary
to prevent a crash.

Signed-off-by: Poonam Aggrwal <poonam.aggrwal@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>