arm: mvebu: turris_omnia: disable MCU watchdog in SPL when booting over UART

When booting over UART, sending U-Boot proper may take too much time and
MCU watchdog will reset the board before U-Boot proper is loaded.

Better disable MCU watchdog in SPL when booting over UART.

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