arm64: dts: imx8mq-librem5: Don't mark buck3 as always on

With the pmic driver fixed we can now shut off the regulator in the gpc.

Signed-off-by: Guido Günther <agx@sigxcpu.org>
Signed-off-by: Martin Kepplinger <martin.kepplinger@puri.sm>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>