regulator: twl-regulator: make TWL4030_ALLOW_UNSUPPORTED more configurable.

regulator: twl-regulator: make TWL4030_ALLOW_UNSUPPORTED more configurable.

The regulators in the twl4030 can provide some voltage settings
that are not offically supported.
These settings are disabled by default, but can be enabled with
  CONFIG_TWL4030_ALLOW_UNSUPPORTED=y
However
 - that config variable is not mentioned in any Kconfig so cannot
   be used, and
 - a global setting is clumsy - a per regulator setting would be
   better.

So define a new 'feature' flag that a board file can set to enable
these unsupported volatages for boards which need them.

This flag cannot (yet) be set using device-tree.

Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>