net: hisilicon: add OF dependency

The HNS MDIO driver fails to build on older ARM machines that are not
yet converted to CONFIG_OF:

drivers/net/ethernet/hisilicon/hns_mdio.c: In function 'hns_mdio_bus_name':
drivers/net/ethernet/hisilicon/hns_mdio.c:405:14: error: 'OF_BAD_ADDR' undeclared (first use in this function)
  u64 taddr = OF_BAD_ADDR;
              ^
drivers/net/ethernet/hisilicon/hns_mdio.c:405:14: note: each undeclared identifier is reported only once for each function it appears in
drivers/net/ethernet/hisilicon/hns_mdio.c:409:11: error: implicit declaration of function 'of_translate_address' [-Werror=implicit-function-declaration]
   taddr = of_translate_address(np, addr);
           ^

This clarifies the dependency to ensure we don't attempt to build these
drivers without CONFIG_OF, but also adds a COMPILE_TEST alternative to
give us better build coverage testing.

Build-tested on x86 as well to ensure this actually works.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>