xilinx: Fix build on x86.

CONFIG_64BIT is not sufficient for checking for availability of
iowrite64() and friends.

Also, the out_addr helpers need to be inline.

Fixes: b690f8df

 ("net: axienet: Use iowrite64 to write all 64b descriptor pointers")
Signed-off-by: David S. Miller <davem@davemloft.net>