arm64: dts: imx8mq-librem5-devkit: Add proximity sensor

Support for the vcnl4040 landet a while ago so add it and the
corresponding pinmux. The irq is currently unused in the driver so don't
configure it yet.

Signed-off-by: Guido Günther <agx@sigxcpu.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>