serial: Add new config option SPL_DEBUG_UART_BASE

SPL_DEBUG_UART_BASE is same as DEBUG_UART_BASE, but applies only for SPL.

In some cases base address of UART is different in SPL and proper U-Boot.

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