arm64: dts: imx8mq-librem5: set audio-1v8 always-on

Unfortunately the codec can't be switched off on it's own. It would ground
the whole bus it's connected to (because of it's built-in diodes to VDD on
i2c pins), making all the other devices on the same bus inaccessible:

[    0.237154] lm3692x 2-0036: Cannot read/clear faults: -11
[    0.242628] lm3692x 2-0036: Fail writing initialization values
[    0.286462] [drm:drm_bridge_attach] *ERROR* failed to attach bridge /soc@0/bus@30800000/mipi-dsi@30a00000 to encoder None-34: -517

Work around this hardware limitation by keeping audio-1v8 enabled.

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