arm: mvebu: turris_omnia: Fix DEBUG_UART_BASE

Internal registers in SPL are at address 0xd0000000 and in proper U-Boot at
address 0xf1000000. UART base address is located in internal registers.

Fix DEBUG_UART_BASE option to correct value for both SPL and proper U-Boot.

This change fixes hangup of proper U-Boot when it is trying to print
something via debug UART.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>