imx8mm_evk: Select the watchdog driver

Currently the watchdog driver is not selected, which causes the following
warnings in both SPL and U-Boot proper:

U-Boot SPL 2020.07-rc1-00387-g67887903af (May 07 2020 - 23:49:27 -0300)
Normal Boot
WDT:   Started without servicing (60s timeout)
Trying to boot from MMC1

U-Boot 2020.07-rc1-00387-g67887903af (May 07 2020 - 23:49:27 -0300)

CPU:   Freescale i.MX8MMQ rev1.0 at 1200 MHz
Reset cause: POR
Model: FSL i.MX8MM EVK board
DRAM:  2 GiB
WDT:   Started without servicing (60s timeout)
....

System reboots after staying 60s in the U-Boot prompt.

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

Suggested-by: Marek Vasut <marex@denx.de>
Signed-off-by: Fabio Estevam <festevam@gmail.com>