m68k: Allow no CPU/platform type for allnoconfig

Allow no CPU/platform type for allnoconfig
  - Provide a dummy value for FPSTATESIZE if no CPU type was selected
  - Provide a dummy value for NR_IRQS if no platform type was selected
  - Warn the user if no CPU or platform type was selected

Note: you still cannot build an allnoconfig kernel, as CONFIG_SWAP=n doesn't
build and we cannot easily fix that
(http://groups.google.com/group/linux.kernel/browse_thread/thread/d430c78b07e1827b)

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>