arm64: dts: imx8mp-tqma8mpql-mba8mpxl: add missing pinctrl for RTC alarm

Although the RTC is on the module, the RTC_EVENT# signal is connected
on the mainboard. Already set by bootloader, but make it explicit in Linux
as well.

Fixes: 418d1d84 ("arm64: dts: freescale: add initial device tree for TQMa8MPQL with i.MX8MP")
Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>