wii: bootwrapper: add fixup to calc useable mem2

The top portion of MEM2 (the second 64MB memory block) in the Nintendo
Wii video game console is used by the firmware running on the Starlet
processor.

Add code to calculate the portion of MEM2 safely useable by the
Broadway processor. When running under the 'mini' firmware this is
easily determined from an in-memory header. Otherwise, a safe default
is used.

Signed-off-by: Albert Herranz <albert_herranz@yahoo.es>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>