ARM: dts: aspeed: ast2600evb: Add MAC0

MAC0 was not functional in the AST2600A0 SoC. This has been resolved
with the A1, so allow use of this port on EVBs with the A1 and
subsequent revisions.

A0 EVBs will still boot with this change, but the first Ethernet device
will not be functional.

Signed-off-by: Joel Stanley <joel@jms.id.au>