ppc: bump SAVE_SPACE to include enough space to store all registers

It looks like SAVE_SPACE at 144 bytes isn't enough to store all the
registers - r29, r30 and r31 end up wrapping the 32 bit stack pointer
around to 0x0.

Fixes: https://github.com/openbios/openbios/issues/27
Signed-off-by: Adrian Chadd <adrian@FreeBSD.org>
Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>