arm64: dts: imx8mq-librem5-r3.dtsi: describe selfie cam XSHUTDOWN pin

The r3 and later revisions of the Librem 5 phone include an additional switch
to control the hi846 XSHUTDOWN pin. Describe it.

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