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>

arch/m68k/lib/cache.c

@@ -132,3 +132,12 @@ void dcache_invalid(void)
 	__asm__ __volatile__("movec %0, %%cacr"::"r"(temp));
 #endif
 }
+
+__weak void invalidate_dcache_range(unsigned long start, unsigned long stop)
+{
+	/* An empty stub, real implementation should be in platform code */
+}
+__weak void flush_dcache_range(unsigned long start, unsigned long stop)
+{
+	/* An empty stub, real implementation should be in platform code */
+}