mmc: sdhci-msm: Add support to scale bus aggregate clock

Bus aggregate clock needs to be scaled according to the power mode in
which the apps clock is running. Add support to read the supported
clock rates of bus aggregate clock and scale this clock based on what
frequency the apps clock is running.

Change-Id: I28fbb1ca50a09fa7753222fe18d2b0774063fc1b
Signed-off-by: Vijay Viswanath <vviswana@codeaurora.org>