verdin-imx8mm: fix watchdog pinctrl issue

Finally, found the root cause of the issue already once mentioned back
here [2] which caused the following error message during boot:

imx_wdt watchdog@30280000:
 pinctrl_select_state_full: uclass_get_device_by_phandle_id: err=-19

Turns out while the watchdog node itself was already u-boot,dm-spl its
pinctrl node was not which caused it to be unavailable at that early
stage. Note that any and all other boards I checked also seem to be
missing this. However, I can't judge whether or not they might indeed
need a similar fix or not.

[2] https://marc.info/?l=u-boot&m=161786572422973



Fixes: commit d304e7ac
 ("ARM: imx8m: Fix reset in SPL on Toradex iMX8MM Verdin")
Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>