serial: Replace CONFIG_DEBUG_UART_BASE by CONFIG_VAL(DEBUG_UART_BASE)

CONFIG_VAL(DEBUG_UART_BASE) expands to CONFIG_DEBUG_UART_BASE or
CONFIG_SPL_DEBUG_UART_BASE or CONFIG_TPL_DEBUG_UART_BASE and allows boards
to set different values for SPL, TPL and U-Boot Proper.

For ns16550 driver this support is there since commit d293759d
("serial: ns16550: Add support for SPL_DEBUG_UART_BASE").

Signed-off-by: Pali Rohár <pali@kernel.org>