regulator: BD71837 support commonly used feedback connection

The BD71837 buck output voltages are in a few cases scaled using external
connection which adds a pull-up to regulator feedback pin. This connection
will adjust output voltage from regulator in a deterministic way.

Add support for describing this HW connection so that driver can adjust
voltage ranges accordingly.

Signed-off-by: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Link: https://lore.kernel.org/r/9959924313db7c7165598604f9a07bf227f471a8.1604994184.git.matti.vaittinen@fi.rohmeurope.com
Signed-off-by: Mark Brown <broonie@kernel.org>