greybus: power_supply: Add runtime pm support

Modify Power_supply greybus driver to support runtime PM framework.

During charging state, the driver will block remote device of suspending,
and then enables runtime suspend when remote device is in none chargin
state.

Testing Done: Compiled and verified on EVT2, EVT2 1x2 GPB test module
              and Device class daughter board.

Signed-off-by: Philip Yang <yang_philip@projectara.com>
Reviewed-by: Rui Miguel Silva <rui.silva@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>