[MIPS] Fix atomic.h build errors.

For the definition of atomic64_t atomic.h was relying on <asm/types.h>
having been included previously.  Before changeset
d89d8e0637a5e4e0a12e90c4bc934d0d4c335239 this was happening as a
side effect of including <linux/spinlock.h>.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>