net: axienet: make the 64b addresable DMA depends on 64b archectures

Currently it is not safe to config the IP as 64-bit addressable on 32-bit
archectures, which cannot perform a double-word store on its descriptor
pointers. The pointer is 64-bit wide if the IP is configured as 64-bit,
and the device would process the partially updated pointer on some
states if the pointer was updated via two store-words. To prevent such
condition, we force a probe fail if we discover that the IP has 64-bit
capability but it is not running on a 64-Bit kernel.

This is a series of patch (1/2). The next patch must be applied in order
to make 64b DMA safe on 64b archectures.

Signed-off-by: Andy Chiu <andy.chiu@sifive.com>
Reported-by: Max Hsu <max.hsu@sifive.com>
Reviewed-by: Greentime Hu <greentime.hu@sifive.com>
Signed-off-by: David S. Miller <davem@davemloft.net>