udoo_neo: 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.

With the watchdog driver model in place, it is no longer needed
to have board code to initialize the watchdog, so remove
its related board code.

Signed-off-by: Fabio Estevam <festevam@gmail.com>