mx6cuboxi: 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@gmail.com>
Tested-by: Christian Gmeiner <cgmeiner@igalia.com>