[MIPS] Tinker with constraints in <asm/atomic.h> to fix build error.

[...]
  CC      init/main.o
include/asm/bitops.h: In function `start_kernel':
include/asm/bitops.h:76: warning: asm operand 2 probably doesn't match
constraints
include/asm/bitops.h:76: warning: asm operand 2 probably doesn't match
constraints
include/asm/bitops.h:76: warning: asm operand 2 probably doesn't match
constraints
include/asm/bitops.h:76: error: impossible constraint in `asm'
include/asm/bitops.h:76: error: impossible constraint in `asm'
include/asm/bitops.h:76: error: impossible constraint in `asm'
make[1]: *** [init/main.o] Error 1
[...]

The build error is caused by the ages old gcc bug where gcc at the time of
analyzing the constraints is unable to figure out that an "i" constraint
actually can be satisfied and thus will abort unless an "r" is added to
the constraint.  For the actual code generation gcc will only ever use the
"i" constraint.

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