twl4030_charger: add software controlled linear charging mode.

   adds a sysfs control node to achieve that.
   It can be set to
   auto: normal automatic charging is enabled (default)
   off: charging is off
   on: charing is on (software controlled)
       CC/CV mode is still automatically done,
       but end of charge due to low current not.

Note: If linear charging mode is used there should be some method of
stopping charging automatically. It is not a so time-critical, but
it is the wrong setting for leaving a charger connected for several days
since Lithium batteries should not be kept at 100% for longer periods.
Linear charging does a good job with not so reliable power sources, since
several voltage controlling is then often too intelligent.
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.
Signed-off-by: Andreas Kemnade <andreas@kemnade.info>