m68k: cache: add an empty stub functions for invalidate/flush dcache

Since some driver like ohci, lcd used dcache functions. But m68k don't
implement the invalidate_dcache_range()/flush_dcache_range() functions.

To avoid compiling errors this patch adds an weak empty stub function
for all m68k cpu.

Also each cpu can implement its own implementation. If not implemented
then by default is using an empty function.

Signed-off-by: Josh Wu <josh.wu@atmel.com>
Acked-by: Angelo Dureghello <angelo@sysam.it>