bq2415x_charger: Use power_supply notifier for automode

This patch removing set_mode_hook function from board data and replacing
it with new string variable of notifier power supply device. After this
change it is possible to add DT support because driver does not need
specific board function anymore. Only static data and name of power supply
device is required.

Signed-off-by: Pali Rohár <pali.rohar@gmail.com>
Reviewed-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Anton Vorontsov <anton@enomsg.org>