Changing the card voltage on the nanopi-k2 is not instantaneous,
especially when switching from 3.3v to 1.8v.

It take at least 3ms for the regulator to go from 3.3v to 1.8v. Add
margin to that to make sure we don't upset the sdcard during the voltage
switch

Fixes: 9bc7ffb0 ("arm64: dts: amlogic: Add NanoPi K2")
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>