[POWERPC] ppc405 Fix arithmatic rollover bug when memory size under 16M

mmu_mapin_ram() loops over total_lowmem to setup page tables.  However, if
total_lowmem is less that 16M, the subtraction rolls over and results in
a number just under 4G (because total_lowmem is an unsigned value).

This patch rejigs the loop from countup to countdown to eliminate the
bug.

Special thanks to Magnus Hjorth who wrote the original patch to fix this
bug.  This patch improves on his by making the loop code simpler (which
also eliminates the possibility of another rollover at the high end)
and also applies the change to arch/powerpc.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>