ARM: dts: msm: move buffer length to main MHI device tree

Main MHI device tree can have the common MHI buffer length
entry instead of having it in overlay device tree nodes so that
the setting stays common.

Change-Id: Ie687c7e2175a54cdaf56938c91e3454872295736
Signed-off-by: Bhaumik Bhatt <bbhatt@codeaurora.org>