msm: kgsl: Add registers and function to read the RBBM_ALWAYSON_COUNTER

Add the appropriate registers to the a4xx register list and add
a function to read the RBBM_ALWAYSON_COUNTER. This counter is
connected to a 19.2 MHz clock and gives us the number of ticks
since the GPU has been on.

Change-Id: Ibc73eb5c3a5c74c39dd483bc16bc8e7f488fa73f
Signed-off-by: Shrenuj Bansal <shrenujb@codeaurora.org>