coresight: enable / disable clocks and regulators during setreg

Currently, hardware event driver keeps the clocks and regulators
ON until user explicitly disables hardware events by echo 0 >
enable. MMSS hardware event clock has dependency on XO clock.
When driver votes for MMSS clock, clocks driver internally turns
XO clock ON. This XO clock vote prevents device to go into XO
shutdown.

Clocks and regulators are needed to program the hardware event
MUXes. They are not needed for generation of hardware events. So
it's logical to enable / disable clocks and regulators only
across these MUX programming.  Move the code to enable / disable
clocks and regulators inside setreg() function.

This patch also removes "enable" device attribute which was used
earlier to enable / disable clocks and regulators required for
programming hardware event MUXes.

Change-Id: Idcca1553deeb0744609d61b4c374743c619b70bb
Signed-off-by: Sarangdhar Joshi <spjoshi@codeaurora.org>