Add include/asm-generic/atomic.h

Add include/asm-generic/atomic.h

The arm, xtensa and mips version of atomic.h were already very similar
(the mips one was a copy of xtensa). Combine these implementations
together to produce a generic atomic.h that can be included by these
architectures (and any others that need it in future).

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>