arm64: dts: freescale: imx8mm-verdin: Fix SD regulator startup delay

The power switch used to power the SD card interface might have
more than 2ms turn-on time, increase the startup delay to 20ms to
prevent failures.

Fixes: 6a57f224 ("arm64: dts: freescale: add initial support for verdin imx8m mini")
Cc: stable@vger.kernel.org
Signed-off-by: Francesco Dolcini <francesco.dolcini@toradex.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>