ARM: dts: imx6qdl-zii-rdu2: Disable the internal RTC

On the imx6qdl-zii-rdu2 board the RTC functionality is provided via
a DS1341 RTC connected via I2C bus, so we can safely disable the internal
one.

Reported-by: Chris Healy <cphealy@gmail.com>
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Tested-by: Chris Healy <cphealy@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>