mx23evk: Convert to CONFIG_DM_SERIAL

The conversion to CONFIG_DM_SERIAL is mandatory, so select this option.

Signed-off-by: Fabio Estevam <festevam@denx.de>
diff --git a/arch/arm/mach-imx/mxs/Kconfig b/arch/arm/mach-imx/mxs/Kconfig
index 0fd4758..6b9d5c9 100644
--- a/arch/arm/mach-imx/mxs/Kconfig
+++ b/arch/arm/mach-imx/mxs/Kconfig
@@ -14,6 +14,7 @@
 
 config TARGET_MX23EVK
 	bool "Support mx23evk"
+	select PL01X_SERIAL
 	select BOARD_EARLY_INIT_F
 
 config TARGET_XFI3