powerpc/mm: Move the STAB0 location to 0x8000 to make room in low memory

Recent upstream builds with allmodconfig fail due to lack of space
between 0x3000 and 0x6000. We have a hard block at 0x7000 but we can
spare a page by moving the STAB0 from 0x6000 to 0x8000.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>