dt-bindings: net: fec: add mx93 description

Add mx93 compatible string for fec driver.

Signed-off-by: Clark Wang <xiaoning.wang@nxp.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>