arm64: dts: librem5-devkit: add the simcom 7100 modem and audio

Add the simcom SIM7100 modem and the sai6 interface that connects it.

Signed-off-by: Angus Ainslie (Purism) <angus@akkea.ca>
Signed-off-by: Martin Kepplinger <martin.kepplinger@puri.sm>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>