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/mach-mtmips/Makefile b/arch/mips/mach-mtmips/Makefile
index 4909b47..19f1e07 100644
--- a/arch/mips/mach-mtmips/Makefile
+++ b/arch/mips/mach-mtmips/Makefile
@@ -1,9 +1,13 @@
 # SPDX-License-Identifier: GPL-2.0+
 
 obj-y += cpu.o
+
+ifneq ($(CONFIG_SOC_MT7621),y)
 obj-y += ddr_init.o
 obj-y += ddr_cal.o
 obj-$(CONFIG_SPL_BUILD) += spl.o
+endif
 
 obj-$(CONFIG_SOC_MT7620) += mt7620/
+obj-$(CONFIG_SOC_MT7621) += mt7621/
 obj-$(CONFIG_SOC_MT7628) += mt7628/