ARM: dts: msm: Update the board subtype for low ram Bengal soc DT

Update the board subtype for low ram Bengal soc DT.
Also remove the platform subtype for 1GB and 2GB soc dtb as it would
make the best match value higher than 4GB soc dtb.

Change-Id: I52e7d0a5ab1f45b0f1c3cbb86b5b43ecf5bc7550