arm64: dts: imx8mq-librem5: describe the voice coil motor for focus control

Describe the focus motor that will be used for the rear camera - even
though the rear camera sensor driver is not yet in the mainline. The
focus motor is a separate device and can be controlled already.

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