net: axienet: add X86 and ARM as supported platforms

This driver should now build on (at least) X86 and ARM platforms, so add
them as supported platforms for the driver in Kconfig.

Signed-off-by: Robert Hancock <hancock@sedsystems.ca>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>