arm: MediaTek: add basic support for MT7629 boards

This adds a general board file based on MT7629 SoCs from MediaTek.

Apart from the generic parts (cpu) we add some low level init codes
and initialize the early clocks.

Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/arm/Makefile b/arch/arm/Makefile
index 4b6c5e1..c38ef3c 100644
--- a/arch/arm/Makefile
+++ b/arch/arm/Makefile
@@ -62,6 +62,7 @@
 machine-$(CONFIG_ARCH_KEYSTONE)		+= keystone
 # TODO: rename CONFIG_KIRKWOOD -> CONFIG_ARCH_KIRKWOOD
 machine-$(CONFIG_KIRKWOOD)		+= kirkwood
+machine-$(CONFIG_ARCH_MEDIATEK)		+= mediatek
 machine-$(CONFIG_ARCH_MESON)		+= meson
 machine-$(CONFIG_ARCH_MVEBU)		+= mvebu
 # TODO: rename CONFIG_TEGRA -> CONFIG_ARCH_TEGRA