mxs: Only build internal Ethernet controller for i.MX28
The internal Ethernet controller is only available on i.MX28
processors so it needs to use CONFIG_MX28 guardian to avoid having
this code called in others.
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Acked-by: Marek Vasut <marex@denx.de>