hwmon: epm_adc: Update timestamp commands

Provide a seperate ioctl interface for obtaining and
setting the PSOC timestamp. Currently we rely on the
clients to set the get/set timestamp command within
the PSOC request command. Unlike other commands
which are obvious, this particular command relies
on the clients. Therefore seperate the command
so that it is obvious and a cleaner approach.

Change-Id: I71915a3e58751cf7c0a961b062c7c06948658e09
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>