MIPS: Add Onion Omega2+ board

MIPS: Add Onion Omega2+ board

The Onion Omega2+ is an MT7688A based board that has 128MB RAM and
multiple peripherals.

The MT7688A is pin compatible with the MT7628A, although the former
supports a 1T1R antenna whereas the MT7628A supports a 2R2T antenna.

Signed-off-by: Harvey Hunt <harvey.hunt@imgtec.com>
Cc: robh+dt@kernel.org
Cc: mark.rutland@arm.com
Cc: john@phrozen.org
Cc: linux-kernel@vger.kernel.org
Cc: devicetree@vger.kernel.org
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/17137/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>