amd-xgbe-phy: Let AMD_XGBE_PHY depend on HAS_IOMEM

The amd-xgbe-phy driver needs to perform ioremap calls, so add HAS_IOMEM
to its build dependency.

Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: David S. Miller <davem@davemloft.net>