regulator: add a new API regulator_set_voltage_tol()

There are some use cases where a voltage range could be reasonably
specified by a target voltage and tolerance.  Add a new API
regulator_set_voltage_tol() wrapping regulator_set_voltage() call
to ease the users.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>