ar71xx: allow to use large ethernet frames on AR934x SoCs

The hardware supports large ethernet frames. Override
the maximum frame length and packet lenght mask in the
platform data to allow to use large MTU on the ethernet
interfaces.

Limit the feature to AR934x SoCs for now. It should work
on some other SoCs as well, but those has not been tested
yet.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>

SVN-Revision: 39149