[new uImage] ppc: Allow boards to specify effective amount of memory

For historical reasons we limited the stack to 256M because some boards
could only map that much via BATS.  However newer boards are capable of
mapping more memory (for example 85xx is capable of doing up to 2G).

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Acked-by: Marian Balakowicz <m8@semihalf.com>