regulator: rk808: Set the enable time for LDOs

The LDOs are documented in the rk808 datasheet to have a soft start
time of 400us.  Add that to the driver.  If this time takes longer on
a certain board the device tree should be able to override with
"regulator-enable-ramp-delay".

This fixes some dw_mmc probing problems (together with other patches
posted to the mmc maiing lists) on rk3288.

Signed-off-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: stable@vger.kernel.org