Microblaze provides its own implementation of atomic_dec_if_positive(),
but nothing else. For a while now, the conditional inc/dec ops have been
optional, and the core code will provide generic implementations using
the code templates in scripts/atomic/fallbacks/.

For simplicity, and for consistency with the other conditional atomic
ops, let's drop the microblaze implementation of
atomic_dec_if_positive(), and use the generic implementation.

With that, we can also drop the local asm/atomic.h and asm/cmpxchg.h
headers, as asm-generic/atomic.h is mandatory-y, and we can pull in
asm-generic/cmpxchg.h via generic-y. This matches what nios2 and nds32
do today.

There should be no functional change as a result of this patch.
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Michal Simek <monstr@monstr.eu>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20210525140232.53872-5-mark.rutland@arm.com