[ARM] mv78xx0: force eth2/eth3 to PHYless mode on pre-A0 silicon

On pre-A0 revisions of the mv78xx0 SoC, the third and fourth
ethernet interface are not brought out to pins, but are internally
cross-connected, so if we run on pre-A0 silicon, we'll force eth2
and eth3 to PHYless mode.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>