MIPS: barrier: Clean up sync_ginv()

Use the new __SYNC() infrastructure to implement sync_ginv(), for
consistency with much of the rest of the asm/barrier.h.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: linux-mips@vger.kernel.org
Cc: Huacai Chen <chenhc@lemote.com>
Cc: Jiaxun Yang <jiaxun.yang@flygoat.com>
Cc: linux-kernel@vger.kernel.org