regulator: core: add regulator_get_linear_step()

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>