greybus: svc: clean up gb_svc struct for pwrmon

The power rail names and counts are unnecessarily stored in the gb_svc
structure once the SVC created, this causes waste of memory usage. This
patch removes rail names and rail counts storage from th gb_svc
structure.

Testing Done:
- Validated the readings from /d/greybus/1-svc/pwrmon/*

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>