twl4030_charger: add software controlled linear charging mode.

Add a 'continuous' option for usb charging which enables
the "linear" charging mode of the twl4030.

Linear charging does a good job with not-so-reliable power sources.
Auto mode does not work well as it switches off when voltage drops
momentarily.  Care must be taken not to over-charge.

It was used with a bike hub dynamo since a year or so. In that case
there are automatically charging stops when the cyclist needs a break.

Original-by: Andreas Kemnade <andreas@kemnade.info>
Signed-off-by: NeilBrown <neil@brown.name>
Signed-off-by: Sebastian Reichel <sre@kernel.org>