Add support for MT7622 reference board

This adds a general board file based on MT7622 SoCs from MediaTek.
This commit is adding the basic boot support for the MT7622 rfb.

Tested-by: Frank Wunderlich <frank-w@public-files.de>
diff --git a/arch/arm/dts/Makefile b/arch/arm/dts/Makefile
index 3c7ecd2..adb843c 100644
--- a/arch/arm/dts/Makefile
+++ b/arch/arm/dts/Makefile
@@ -872,6 +872,7 @@
 			      k3-j721e-r5-common-proc-board.dtb
 
 dtb-$(CONFIG_ARCH_MEDIATEK) += \
+	mt7622-rfb.dtb \
 	mt7623n-bananapi-bpi-r2.dtb \
 	mt7629-rfb.dtb \
 	mt8512-bm1-emmc.dtb \