msm: board-7627a: Feed in reserved sizes via variable

ION reservation at boot-time is subject to change via
early parameter or dynamically at runtime. So, let's
not define all the sizes rigidly via macros, instead
use variables.

Change-Id: I46ac6ad2365e45de264041cf11a6c9daba837779
Signed-off-by: Chintan Pandya <cpandya@codeaurora.org>