ARM64: dts: meson-gxbb: p20x: add card regulator settle times

Changing the card voltage on the p200 is not instantaneous, especially
when switching from 3.3v to 1.8v.

I take at least 70ms 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: ef8d2ffe

 ("ARM64: dts: meson-gxbb: add MMC support")
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>