MIPS: dts: ralink: Add Mediatek MT7628A SoC

MIPS: dts: ralink: Add Mediatek MT7628A SoC

The MT7628A is the successor to the MT7620 and pin compatible with the
MT7688A, although the latter supports only a 1T1R antenna rather than
a 2T2R antenna.

This commit adds support for the following features:

- UART
- USB PHY
- EHCI
- Interrupt controller
- System controller
- Memory controller
- Reset controller

Signed-off-by: Harvey Hunt <harvey.hunt@imgtec.com>
Cc: robh+dt@kernel.org
Cc: mark.rutland@arm.com
Cc: john@phrozen.org
Cc: Harvey Hunt <harvey.hunt@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: devicetree@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: linux-mediatek@lists.infradead.org
Patchwork: https://patchwork.linux-mips.org/patch/17133/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>