board: ten64: disable watchdog autostart

board: ten64: disable watchdog autostart

The watchdog driver was previously enabled but not used
until U-Boot's fsl-ls1088a.dtsi was updated to describe them.

Some Linux distributions (e.g Debian 11) do not engage the
SP805 watchdogs, causing unexpected resets after boot.

To conserve the user experience, turn off the autostart,
and we will provide a mechanism to turn them on at boot
via env vars.

Signed-off-by: Mathew McBride <matt@traverse.com.au>
Reviewed-by: Peng Fan <peng.fan@nxp.com>