mips: ath79: add AP121 reference board

This patch add board-level code and base DT for AP121.

Signed-off-by: Wills Wang <wills.wang@live.com>
[updated defconfig, enabled CONFIG_USE_PRIVATE_LIBGCC=y]
Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
diff --git a/arch/mips/mach-ath79/Kconfig b/arch/mips/mach-ath79/Kconfig
index e35f1b5..c3012f8 100644
--- a/arch/mips/mach-ath79/Kconfig
+++ b/arch/mips/mach-ath79/Kconfig
@@ -22,4 +22,15 @@
 	help
 	  This supports QCA/Atheros qca953x family SOCs.
 
+choice
+	prompt "Board select"
+
+config TARGET_AP121
+	bool "AP121 Reference Board"
+	select SOC_AR933X
+
+endchoice
+
+source "board/qca/ap121/Kconfig"
+
 endmenu