ARM: dts: msm: add clock scaling for SDHC device for 8996

SDHC provides standard host interface to SD/MMC with various
frequencies supported.
As part of clock scaling frequencies toggle this
change provides the supported values for clock scaling.
The clock scaling logic shall toggle between these values.

Change-Id: I8d6713d575d6a42542780ea9ed3d7f7dda094beb
Signed-off-by: Talel Shenhar <tatias@codeaurora.org>