arm64: dts: imx8mq-librem5: Describe MIC_2V4 regulator

No functional change, but it describes the hardware better.

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>