regulator: bd718x7: Support external connection to scale voltages

Setups where regulator (especially the buck8) output voltage is scaled
by adding external connection where some other regulator output is
connected to feedback-pin (over suitable resistors) is getting popular
amongst users of BD71837. This allows for example scaling down the
buck8 voltages to suit lover GPU voltages for projects where buck8 is
(ab)used to supply power for GPU. As a note - some setups do allow DVS
for buck8. This do produce voltage spikes and the HW must be evaluated
to be able to survive them. Thus this commit still keep the DVS disabled
for non DVS bucks by default. Let's not help you burn your proto board.

Allow describing this external connection from DT and scale the
voltages accordingly. This is what the connection should look like:

|------------|
|    buck 8  |-------+----->Vout
|            |       |
|------------|       |
     | FB pin        |
     |               |
     +-------+--R2---+
             |
             R1
             |
     V FB-pull-up

     Here the buck output is sifted according to formula:

Vout_o = Vo - (Vpu - Vo)*R2/R1
Linear_step = step_orig*(R1+R2)/R1

where:
Vout_o is adjusted voltage output at vsel reg value 0
Vo is original voltage output at vsel reg value 0
Vpu is the pull-up voltage V FB-pull-up in the picture
R1 and R2 are resistor values.

Bring support for specifying the Vpu, R1 and R2 from device tree and
scale voltages if they are given.

Signed-off-by: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Link: https://lore.kernel.org/r/89b2be87074f307a8823f15f34e1f662023cbf36.1604994184.git.matti.vaittinen@fi.rohmeurope.com
Signed-off-by: Mark Brown <broonie@kernel.org>