net: mvneta: enable setting custom TX IP checksum limit

Since Armada 38x SoC can support IP checksum for jumbo frames only on
a single port, it means that this feature should be enabled per-port,
rather than for the whole SoC.

This patch enables setting custom TX IP checksum limit by adding new
optional property to the mvneta device tree node. If not used, by
default 1600B is set for "marvell,armada-370-neta" and 9800B for other
strings, which ensures backward compatibility. Binding documentation
is updated accordingly.

Signed-off-by: Marcin Wojtas <mw@semihalf.com>
Signed-off-by: David S. Miller <davem@davemloft.net>