sh: Missing flush_dcache_all() proto in cacheflush.h.

Some boards need this, so provide a definition.
Signed-off-by: NPaul Mundt <lethal@linux-sh.org>

include/asm-sh/cpu-sh4/cacheflush.h

@@ -17,6 +17,7 @@
  *  so we need them.
  */
 void flush_cache_all(void);
+void flush_dcache_all(void);
 void flush_cache_mm(struct mm_struct *mm);
 #define flush_cache_dup_mm(mm) flush_cache_mm(mm)
 void flush_cache_range(struct vm_area_struct *vma, unsigned long start,