arm: mvebu: turris_omnia: disable MCU watchdog in board_late_init()

Disable MCU watchdog in board_late_init() instead of board_init(), so
that it is disabled after U-Boot enables SOC watchdog instead of before.
This way there is no window when the board is vulnerable.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>