x86/config: Revamp configuration for MID devices

This follows on from the patch applied in 3.2rc1 which creates
an INTEL_MID configuration. We can now add the entry for
Medfield specific code. After this is merged the final patch
will be submitted which moves the rest of the device Kconfig
dependancies to MRST/MEDFIELD/INTEL_MID as appropriate.

Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>