drivers: serial: DEBUG_UART_SKIP_INIT depends on DEBUG_UART

DEBUG_UART_SKIP_INIT is used only by debug UART and thus should depend
on DEBUG_UART.

Signed-off-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/serial/Kconfig b/drivers/serial/Kconfig
index b7ff296..e67ea52 100644
--- a/drivers/serial/Kconfig
+++ b/drivers/serial/Kconfig
@@ -442,6 +442,7 @@
 
 config DEBUG_UART_SKIP_INIT
 	bool "Skip UART initialization"
+	depends on DEBUG_UART
 	help
 	  Select this if the UART you want to use for debug output is already
 	  initialized by the time U-Boot starts its execution.