pico-pi-imx7d: Convert to watchdog driver model

Commit 68dcbdd5 ("ARM: imx: Add weak default reset_cpu()") caused
the 'reset' command in U-Boot to not cause a board reset.

Fix it by switching to the watchdog driver model via sysreset, which
is the preferred method for implementing the watchdog reset

Signed-off-by: Fabio Estevam <festevam@denx.de>