net: ethernet: mediatek: support MT7621 SoC ethernet hardware

The Mediatek MT7621 SoC contains the same ethernet hardware module as
used on a number of other MediaTek SoC parts. There are some minor
differences to deal with but we can use the same driver to support
them all.

This patch is based on work by Bjørn Mork <bjorn@mork.no>, and his
original patch is at:

https://github.com/bmork/LEDE/commit/3293bc63f5461ca1eb0bbc4ed90145335e7e3404



There is an additional compatible devicetree type added, and the primary
change to the code required is to support a single interrupt (for both
RX and TX interrupts).

Signed-off-by: Bjørn Mork <bjorn@mork.no>
[gerg@kernel.org: rebase to mainline and irq handler fix]
Signed-off-by: Greg Ungerer <gerg@kernel.org>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Sean Wang <sean.wang@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>