regulator: s2mps11: Convert ramp rate to uV/us and set default ramp rate

This patch makes driver to use uV/us as units of ramp_delay. It makes driver
in compliance with regulator framework and make ramp rate precise.

This patch also sets default ramp rate in regulator descriptor which can be
used in case if case ramp rate is not set in regulator constraints.

Signed-off-by: Yadwinder Singh Brar <yadi.brar@samsung.com>
Signed-off-by: Mark Brown <broonie@linaro.org>