net: amd-xgbe: Add hardware dependency

The amd-xgbe driver currently only works with the Seattle SoC, which
is ARM64 architecture, so there is no point in building this driver on
other architectures except for build testing purpose. The dependency
list can be updated later if the driver ever supports other
architectures.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>