ARM: dts: msm: update sdhci-msm device tree documentation

The bus-speed-mode binding for sdhci-msm supports HS400 and while
the driver has been updated to reflect this, the documentation has
not been updated accordingly.

Change-Id: I55ba444150f2523197185bd371bf500efd96b9ef
Signed-off-by: Krishna Konda <kkonda@codeaurora.org>