greybus: power_supply: add callback to handle power supply changes

When checking for property changes we may need to act upon that change
besides reporting it using power_supply_changed. So, add a function that
will be call if the specific property changed.

As at it, adjust some indentation of the psy_props_changes array.

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