arm64: dts: imx8mm-var-som-symphony: Drop wake-up source from RTC

The RTC on Symphony board does not have its interrupt pin connected to
the SoC, therefore it is not capable of waking up.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>