net: cadence: Add architecture dependencies

The Cadence ethernet chipsets are only used on specific ARM
architectures. Add Kconfig dependencies so that drivers for these
chipsets are only buildable on the relevant architectures.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>