msm: rtb: Add timestamp to rtb logging

RTB logging currently doesn't log the time
at which the logging was done. This can be
useful to compare with dmesg during debug.
The bytes for timestamp are taken by reducing
the sentinel array size to three from eleven
thus giving the extra 8 bytes to store time.
This maintains the size of the layout at 32.

Change-Id: Ifc7e4d2e89ed14d2a97467891ebefa9515983630
Signed-off-by: Vignesh Radhakrishnan <vigneshr@codeaurora.org>