twl4030_charger: Add backup-battery charging

This allows a voltage and current (bb_uvolts and bb_uamps) to be
specified in the platform_data, and charging of the backup battery will
be enabled with those specification.

As it is not possible to monitor the backup battery at all there is no
new device created to represent it.

Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>