mmc: sdhci-msm: get lower bus speed mode for clock scaling

The lower bus speed mode to be used during clock scaling may
vary based on the target. Hence, add a new dtsi property to
define this bus speed mode.

Change-Id: If8e2d125b8246ca479f816a475940bb357138297
Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>
Signed-off-by: Asutosh Das <asutoshd@codeaurora.org>