msm_rtb: Add snapshot of msm_rtb.h

This is a snapshot of msm_rtb.h file taken as of msm-4.19
commit <cf8acfa8c9b53ad> (Merge "msm: camera: core : Validate
the dev name during the node ioctl handler").

Change-Id: Ia2a2147369328ca6da177bd51c20217df3c4f157
Signed-off-by: Raghavendra Rao Ananta <rananta@codeaurora.org>