defconfig: msm: Reduce common CMA area to 8MB

Limit common CMA region to 8MB for 64 bit target
config to align it with 32 bit config.

Change-Id: I127abcbebfd31add6eebc5d6685fa606223b920f
Signed-off-by: Susheel Khiani <skhiani@codeaurora.org>
Signed-off-by: Chintan Pandya <cpandya@codeaurora.org>