bindings: Add qcom,msm-rtb schema

Add msm_rtb.yaml from a converted msm_rtb.txt from
commit <baa63fa212bd> (Merge "ARM: dts: msm: Add multiple refresh
 rates for sw43404") to new yaml format.

Change-Id: I79e61b02578c27048c406af6ee493fab1718a1d4