mpc5200: Fix Kconfig dependancies on MPC5200 FEC device driver

When not building an arch/powerpc kernel, the mpc5200 FEC driver depends
on some symbols which are not defined (BESTCOMM & BESTCOMM_FEC).

This patch flips around the dependancy logic so that it cannot be
selected unless BESTCOMM_FEC is selected first.  Kconfig stops
complaining this way.

Also, the driver only works for arch/powerpc (not arch/ppc) anyway so
it should depend on PPC_MERGE also.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Jeff Garzik <jeff@garzik.org>