dt-bindings: mips: Add bindings for TP-Link HC220 G5 v1 board

Add bindings for the compatible string used for the TP-Link's
HC220 G5 V1 board, a wireless AP based on MT7621.

Signed-off-by: Liviu Dudau <liviu@dudau.co.uk>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Acked-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>