[POWERPC] 4xx: update 440EP(x)/440GR(x) identical PVR issue workaround

Renaming the CPU nodes with generic names put the CPU model in
the "model" property and thus broke the PowerPC 440EP(x)/440GR(x)
identical PVR workaround. The updates it to use the new model property
for CPU identification.

Signed-off-by: Valentine Barshak <vbarshak@ru.mvista.com>
Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>