greybus: svc: add AP power measurements debugfs support

This change adds the AP Power Monitor functions to
read out all the rails power information monitored by
the SVC.

Testing Done:
- $ cat /d/greybus/1-svc/pwrmon/*/*
  and validate the output with the svc stub power
  monitor functions
- $ tree /d/greybus/1-svc/pwrmon
  | | | |---pwrmon
  | | | | |---DUMMY_RAIL_1
  | | | | | |---current_now
  | | | | | |---power_now
  | | | | | |---voltage_now
  | | | | |---DUMMY_RAIL_2
  | | | | | |---current_now
  | | | | | |---power_now
  | | | | | |---voltage_now
  | | | | |---DUMMY_RAIL_3
  | | | | | |---current_now
  | | | | | |---power_now
  | | | | | |---voltage_now
  | | | | |---DUMMY_RAIL_4
  | | | | | |---current_now
  | | | | | |---power_now
  | | | | | |---voltage_now

Signed-off-by: David Lin <dtwlin@google.com>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>