regulator: bd9576 add FET ON-resistance for OCW

BD9576MUF provides over-current protection and detection. Current is
measured as voltage loss over external FET. Allow specifying FET's on
resistance so current monitoring limits can be converted to voltages.

Signed-off-by: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/e5feb160d7e09f33fff5b88f1928c66a15c6680f.1622628334.git.matti.vaittinen@fi.rohmeurope.com


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