arm64: mm: fix the function name in comment of __flush_dcache_area

Fix the function name of comment of __flush_dcache_area,
because __flush_dcache_area is the correct name. Also,
the missing variable 'size' is added to the comment.
Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/mm/cache.S

@@ -146,7 +146,7 @@ ENDPROC(flush_icache_range)
 ENDPROC(__flush_cache_user_range)
 
 /*
- *	__flush_kern_dcache_page(kaddr)
+ *	__flush_dcache_area(kaddr, size)
  *
  *	Ensure that the data held in the page kaddr is written back to the
  *	page in question.