twl4030_charger: Add devicetree support

This allows the charger to be enabled with devicetree, and allows the
parameters for charging the backup battery to be set.

Signed-off-by: NeilBrown <neilb@suse.de>
Acked-by: Kumar Gala <galak@codeaurora.org>
Acked-by: Grant Likely <grant.likely@linaro.org>
Signed-off-by: Anton Vorontsov <anton@enomsg.org>