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 d293759d55cc
("serial: ns16550: Add support for SPL_DEBUG_UART_BASE").

Signed-off-by: Pali Rohár <pali@kernel.org>
diff --git a/drivers/serial/serial_mtk.c b/drivers/serial/serial_mtk.c
index 4145d9f..a84f39b 100644
--- a/drivers/serial/serial_mtk.c
+++ b/drivers/serial/serial_mtk.c
@@ -426,7 +426,7 @@
 {
 	struct mtk_serial_priv priv;
 
-	priv.regs = (void *) CONFIG_DEBUG_UART_BASE;
+	priv.regs = (void *) CONFIG_VAL(DEBUG_UART_BASE);
 	priv.clock = CONFIG_DEBUG_UART_CLOCK;
 
 	writel(0, &priv.regs->ier);
@@ -439,7 +439,7 @@
 static inline void _debug_uart_putc(int ch)
 {
 	struct mtk_serial_regs __iomem *regs =
-		(void *) CONFIG_DEBUG_UART_BASE;
+		(void *) CONFIG_VAL(DEBUG_UART_BASE);
 
 	while (!(readl(&regs->lsr) & UART_LSR_THRE))
 		;