imx8mm_beacon: Select the watchdog driver

Currently watchdog driver is not selected, which causes system to reboot
after staying 60s in the U-Boot prompt.

Fix this problem by enabling CONFIG_WATCHDOG so that watchdog can be
properly serviced.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Adam Ford <aford173@gmail.com>