[PATCH] powerpc: generalize PPC44x_PIN_SIZE

The following patch generalizes PPC44x_PIN_SIZE by changing it to
PPC_PIN_SIZE, which can be defined by any sub-arch to automatically adjust
VMALLOC_START.

Define PPC_PIN_SIZE on 8xx, avoiding potential conflicts with the
pinned space.

Signed-off-by: Marcelo Tosatti <marcelo.tosatti@cyclades.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>