MIPS: dts: Add Vocore2 board

The VoCore2 board is a low cost MT7628A based board with 128MB RAM, 16MB
flash and multiple external peripherals.

This initial DTS provides enough support to get to userland and use the USB
port.

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/17134/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>