ion: msm: move custom zero'ing functions out of core Ion

MSM targets currently use some optimized zero'ing functions. However,
there are always merge conflicts when bringing in patches from upstream
so these custom functions should be moved into the MSM space and the
original zero'ing functions from upstream should be restored.

All users of the custom zero'ing functions should continue to use the
custom functions. The goal, over time, is to unify the approaches but
for now just leave them separated.

Change-Id: I5a545af2358d8a7a20737b30d181b3057407a831