regulator: da9052: Add ramp speed information for the DVC regulators

Some LDOs and DCDCs have a fixed ramp speed of 6.25 mV/µs. This patch adds
the set_voltage_time_sel function to let consumers know about this.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Mark Brown <broonie@linaro.org>