arm64: dts: imx8mq-librem5: Reduce usdhc2's post-power-on-delay-ms to 20ms

This appears to be enough for both Redpine and SparkLAN cards.
Waiting for too long makes us waste time in resume from system
suspend, so let's keep it as short as possible.

Signed-off-by: Sebastian Krzyszkowiak <sebastian.krzyszkowiak@puri.sm>
Signed-off-by: Martin Kepplinger <martin.kepplinger@puri.sm>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>