msm: kgsl: Add support to configure bus dcvs sampling interval

Add support to configure bus dcvs sampling interval through the
sysfs entry. Also expose cur_ab vote value to sysfs.

$ echo 25000 > /sys/class/devfreq/soc:qcom,kgsl-busmon/sampling_interval
$ cat /sys/class/devfreq/soc:qcom,kgsl-busmon/cur_ab .

Change-Id: I77df55de2766d3fd818c27f717cfc4ae598bb0bd
Signed-off-by: Hareesh Gundu <hareeshg@codeaurora.org>