staging: mt7621-eth: add support for mt7621

Add support for SoCs from the mt7621 family. These all have 2 GMAC ports,
both of which are attached to the same internal 1000MBit switch. Currently
we only support GMAC1 as the sole CPU port. MT7621 is very similar to
MT7620 with only a few registers having different offsets. MT7621 is the
first SoC to have the new QDMA engine builtin. The older PDMA engine is
also present. unfortunatley, to get the best performance we need to run RX
on PDMA and TX on QDMA. This SoC is also the first to have TX vlan
offloading and TSO6 support.

NeilBrown: the driver didn't work when I tested, so I changed it
  to match known-working code as much as possible.  This included
  converting to the PDMA engine for TX.

Signed-off-by: John Crispin <blogic@openwrt.org>
Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Michael Lee <igvtee@gmail.com>
Signed-off-by: NeilBrown <neil@brown.name>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>