ftgmac100: Use device "compatible" property, not machine.

We test for aspeed chips to handle a couple of special cases,
but we do that by checking the machine type which isn't right.

Instead check the actual device compatible property. This also
updates the dtsi files for the aspeed SoC to match.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: David S. Miller <davem@davemloft.net>