greybus: svc: add Interface power measurements support

greybus: svc: add Interface power measurements support

This change implements the AP Power Monitor functions for obtaining
current/voltage/power on a specific rail of an Interface.

Testing Done:

$ cat /sys/bus/greybus/devices/1-3/current_now
103
$ cat /sys/bus/greybus/devices/1-3/power_now
303
$ cat /sys/bus/greybus/devices/1-3/voltage_now
203

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