ramips: mt7621: add Mediatek AP-MT7621A-V60 EVB support

This board has:
- mt7621 SoC
- 8MB SPI flash
- 128MB RAM
- 5x ethernet ports from internal (SoC) switch
- 1x ethernet port sitting on gmac2 and IC+ phy (not yet supported)
- 3x PCIe slots
- 1x USB 2.0 and 1x USB 3.0
- sound based on wm8960
- SDXC card slot (full size)

First fw write from interactive u-boot menu, interrupt with 2.
After that sysupgrade.

Tested both with 4.9 and 4.14

Signed-off-by: Roman Yeryomin <roman@advem.lv>

target/linux/ramips/base-files/lib/upgrade/platform.sh

@@ -23,6 +23,7 @@ platform_check_image() {
 	all0256n-8M|\
 	all5002|\
 	all5003|\
+	mediatek,ap-mt7621a-v60|\
 	ar725w|\
 	asl26555-8M|\
 	asl26555-16M|\