serial: Support debug UART in TPL

Some boards want to use the debug UART in TPL so add an option for that.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/serial/Kconfig b/drivers/serial/Kconfig
index 3a1e5a6..8b27ad9 100644
--- a/drivers/serial/Kconfig
+++ b/drivers/serial/Kconfig
@@ -539,6 +539,13 @@
 	help
 	  This is the base address of your UART for memory-mapped UARTs for TPL.
 
+config VPL_DEBUG_UART_BASE
+	hex "Base address of UART for VPL"
+	depends on VPL && DEBUG_UART
+	default DEBUG_UART_BASE
+	help
+	  This is the base address of your UART for memory-mapped UARTs for VPL.
+
 config DEBUG_UART_CLOCK
 	int "UART input clock"
 	depends on DEBUG_UART