m68k: only generate FPU instructions if CONFIG_FPU enabled

Most of the m68k code that supports a hardware FPU is surrounded by
CONFIG_FPU. Be consistent and surround the hardware FPU instruction
setup in setup_mm.c with CONFIG_FPU as well as the check for
CONFIG_M68KFPU_EMU_ONLY.

The existing classic m68k architectures all define CONFIG_FPU, so they
see no change from this. But on ColdFire where we do not support the
emulated FP code we can now compile without CONFIG_FPU being set as well.

Signed-off-by: Greg Ungerer <gerg@linux-m68k.org>