arm64: dts: imx8mq-librem5-devkit: Disable snvs_rtc

The board has it's own RTC chip which is backed by the (optional)
battery and hence preserves data/time on poweroff when that is inserted.

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