imx8mn_ddr4_evk: 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: NFabio Estevam <festevam@gmail.com>

configs/imx8mn_ddr4_evk_defconfig

@@ -79,5 +79,4 @@ CONFIG_SPL_SYSRESET=y
 CONFIG_SYSRESET_PSCI=y
 CONFIG_SYSRESET_WATCHDOG=y
 CONFIG_DM_THERMAL=y
-# CONFIG_WATCHDOG is not set
 CONFIG_IMX_WATCHDOG=y