arm: mvebu: Enable DM_SERIAL on AXP / A38x boards

This patch enables DM_SERIAL for all ARCH_MVEBU boards (AXP & A38x).

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Simon Glass <sjg@chromium.org>
Cc: Luka Perkov <luka.perkov@sartura.hr>
diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 50322c4..a3bd845 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -112,6 +112,7 @@
 	select OF_CONTROL
 	select OF_SEPARATE
 	select DM
+	select DM_SERIAL
 
 config TARGET_DEVKIT3250
 	bool "Support devkit3250"