ARM: dts: msm: update documentation for rpm-smd property.

Meaning of rpm-standalone property got changed after
I102b06420fdfa1505d8f43e8936a8e9cee4f1ce4. Updated the
documentation as per latest usage.

Change-Id: I067527b31db102d09be39324c9e656ed741798c5
Signed-off-by: Anji Jonnala <anjir@codeaurora.org>