ARM: dts: msm: Add common CMA device configuration for sdx20

TZ expects firmware regions under 256MB. BG firmware region is
allocated from common CMA region. Add common CMA configuration
and restrict it within 256MB region for sdx20.

Change-Id: I0b041520948878bc73719d84ab0ed0099fb7a54f
Signed-off-by: Vishwanath Raju K <vishk@codeaurora.org>