mips: mtmips: add support for MediaTek MT7621 SoC

This patch adds support for MediaTek MT7621 SoC.
All files are dedicated for u-boot.

The default build target is u-boot-mt7621.bin.

The specification of this chip:
https://www.mediatek.com/products/homenetworking/mt7621

Reviewed-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>
diff --git a/arch/mips/Makefile b/arch/mips/Makefile
index 32c436f..a007914 100644
--- a/arch/mips/Makefile
+++ b/arch/mips/Makefile
@@ -4,6 +4,11 @@
 
 ifeq ($(CONFIG_SPL_BUILD),y)
 head-$(CONFIG_ARCH_JZ47XX) := arch/mips/mach-jz47xx/start.o
+head-$(CONFIG_SOC_MT7621) := arch/mips/mach-mtmips/mt7621/spl/start.o
+endif
+
+ifeq ($(CONFIG_TPL_BUILD),y)
+head-$(CONFIG_SOC_MT7621) := arch/mips/mach-mtmips/mt7621/tpl/start.o
 endif
 
 libs-y += arch/mips/cpu/