ARM: msm: flo: add limit_mem= kernel command line parameter

Add a command line parameter to limit available memory (after
all carveouts are reserved) to the specified size.  This can
be used to help test low memory situations.

Change-Id: Ia25e028315260b706365afe820e6e9986e8e7e2d
Signed-off-by: Colin Cross <ccross@android.com>
Signed-off-by: Iliyan Malchev <malchev@google.com>