arm: MediaTek: add basic support for MT7623 boards

This adds a general board file based on MT7623 SoCs from MediaTek.

As this u-boot is loaded by MTK proprietary preloader, there is no
low level initializtion codes.

Tested-by: Matthias Brugger <matthias.bgg@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/arm/mach-mediatek/Makefile b/arch/arm/mach-mediatek/Makefile
index 852d330..b5d3a37 100644
--- a/arch/arm/mach-mediatek/Makefile
+++ b/arch/arm/mach-mediatek/Makefile
@@ -3,4 +3,5 @@
 obj-y	+= cpu.o
 obj-$(CONFIG_SPL_BUILD)	+= spl.o
 
+obj-$(CONFIG_TARGET_MT7623) += mt7623/
 obj-$(CONFIG_TARGET_MT7629) += mt7629/