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.comSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/regulator/rohm,bd71837-regulator.yaml

@@ -105,6 +105,54 @@ patternProperties:
           PMIC hardware state machine.
         type: boolean
 
+      # 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.
+      #
+      # So we allow describing this external connection from DT and scale the
+      # voltages accordingly. This is what the connection should look like:
+      #
+      # |---------------|
+      # |       buck 8  |-------+----->Vout
+      # |               |       |
+      # |---------------|       |
+      #        |                |
+      #        |                |
+      #        +-------+--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.
+
+      rohm,fb-pull-up-microvolt:
+        description:
+          Feedback-pin has pull-up connection to adjust voltage range. This is
+          the used pull-up voltage before R1.
+
+      rohm,feedback-pull-up-r1-ohms:
+        description:
+          Feedback-pin has pull-up connection to adjust voltage range. This is
+          the used R1 resistor.
+
+      rohm,feedback-pull-up-r2-ohms:
+        description:
+          Feedback-pin has pull-up connection to adjust voltage range. This is
+          the used R2 resistor.
+
     required:
       - regulator-name