net: mvneta: Add support for AlleyCat5

Add support for the AlleyCat5 SoC. This lacks the mbus from the other
users of the mvneta.c driver so a new compatible string is needed to
allow for a different window configuration.

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>