x86/intel_mid: Fix the Kconfig for MID selection

We currently fail to build on CONFIG_X86_INTEL_MID=y and
CONFIG_X86_MRST unset.

We could build all the bits to make generic MID work if you
picked MID platform alone but that's really silly. Instead use
select and two variables.

This looks a bit daft right now but once we add a Medfield
selection it'll start to look a good deal more sensible.

Reported-by: Ingo Molnar <mingo@elte.hu>
Reported-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Link: http://lkml.kernel.org/r/20111205231433.28811.51297.stgit@bob.linux.org.uk
Signed-off-by: Ingo Molnar <mingo@elte.hu>