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>