net: fsl: Allow most drivers to be built with COMPILE_TEST

There are only a handful of Freescale Ethernet drivers that don't
actually build with COMPILE_TEST:

* FEC, for which we would need to define a default register layout if no
  supported architecture is defined

* UCC_GETH which depends on PowerPC cpm.h header (which could be moved
  to a generic location)

* GIANFAR needs to depend on HAS_DMA to fix linking failures on some
  architectures (like m32r)

We need to fix an unmet dependency to get there though:
warning: (FSL_XGMAC_MDIO) selects OF_MDIO which has unmet direct
dependencies (OF && PHYLIB)

which would result in CONFIG_OF_MDIO=[ym] without CONFIG_OF to be set.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>