m68k: make support for FPU hardware configurable

The classic m68k code has always supported an FPU (although it may have
been a software emulated one). The non-MMU m68k code has never supported FPU
hardware. To help in merging common code create a configation setting that
signifies if we are builing in FPU support or not.

This switch, CONFIG_FPU, is set as per the current use cases. So it is
always enabled if CONFIG_MMU is set, and disabled otherwise. With a little
extra code it will be possible to disable it on the classic m68k platforms
as well, and to enable it on non-MMU platforms that do have hardware FPU.

Signed-off-by: Greg Ungerer <gerg@uclinux.org>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>