net: mvneta: Add naive RSS support

This patch adds the support for the RSS related ethtool
function. Currently it only uses one entry in the indirection table which
allows associating an mvneta interface to a given CPU.

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Tested-by: Marcin Wojtas <mw@semihalf.com>
Signed-off-by: David S. Miller <davem@davemloft.net>