slim:msm: Change to register access for RX path if required

MSM controller for slimbus usually uses a BAM pipe to receive control
data. However, some targets may not have a BAM pipe assinged for
receiving this data. In that case, RX data should be received using
direct register access instead.
Introduce a Device-Tree property to change this access.

Change-Id: I9bb1502f25e0e855937bc6fb4131f55348d04794
Signed-off-by: Sagar Dharia <sdharia@codeaurora.org>