msm: memory: Calculate DDR size early at boot time

Calculate the DDR size available to APPS which can be
used during early boot up time such as reservation
size based on the available RAM.

Change-Id: Ib765913ed95f8c23e4db3c044e5a43bcc10b7c1c
Signed-off-by: Chintan Pandya <cpandya@codeaurora.org>