Add regulator_get_linear_step(), which returns the voltage step size
between VSEL values for linear regulators. This is intended for use
by regulator consumers which build their own voltage-to-VSEL tables.
Signed-off-by: Paul Walmsley <pwalmsley@nvidia.com> Reviewed-by: Andrew Chew <achew@nvidia.com> Cc: Matthew Longnecker <mlongnecker@nvidia.com> Signed-off-by: Mark Brown <broonie@linaro.org>