mips: mtmips: add support for mt7628-rfb

This patch adds support for mt7628 reference board. SPL_DM and DT are not
enabled for SPL to save about 17KiB for u-boot-spl.bin.

Reviewed-by: Stefan Roese <sr@denx.de>
diff --git a/board/mediatek/mt7628/Makefile b/board/mediatek/mt7628/Makefile
new file mode 100644
index 0000000..db129c5
--- /dev/null
+++ b/board/mediatek/mt7628/Makefile
@@ -0,0 +1,3 @@
+# SPDX-License-Identifier: GPL-2.0
+
+obj-y += board.o