board: amlogic: add support for S400 board

The S400 board is the Amlogic AXG SoC reference board including :
 - Amlogic A113DX ARM Cortex-A53 quad-core SoC @ 1.2GHz
 - 1GB DDR4 SDRAM
 - 10/100 Ethernet
 - 2 x USB 2.0 Host
 - eMMC
 - Infrared receiver
 - SDIO WiFi Module
 - MIPI DSI Connector
 - Audio HAT Connector
 - PCI-E M.2 Connectors

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
diff --git a/arch/arm/mach-meson/Kconfig b/arch/arm/mach-meson/Kconfig
index 7f217fa..deddcc5 100644
--- a/arch/arm/mach-meson/Kconfig
+++ b/arch/arm/mach-meson/Kconfig
@@ -63,6 +63,7 @@
 	default "odroid-c2" if MESON_GXBB
 	default "p212" if MESON_GXL
 	default "q200" if MESON_GXM
+	default "s400" if MESON_AXG
 	default ""
 	help
 	  This option contains information about board name.