powerpc/8xx: Add support for TASK_SIZE greater than 0x80000000

By default, TASK_SIZE is set to 0x80000000 for PPC_8xx, which is most
likely sufficient for most cases. However, kernel configuration allows
to set TASK_SIZE to another value, so the 8xx shall handle it.

This patch also takes into account the case of PAGE_OFFSET lower than
0x80000000, allthought most of the time it is equal to 0xC0000000

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Scott Wood <scottwood@freescale.com>