locking/atomic: openrisc: avoid asm-generic/atomic.h

OpenRISC is the only architecture which uses asm-generic/atomic.h and
also provides its own implementation of some functions, requiring
ifdeferry in the asm-generic header. As OpenRISC provides the vast
majority of functions itself, it would be simpler overall if it also
provided the few functions it cribs from asm-generic.

This patch decouples OpenRISC from asm-generic/atomic.h. Subsequent
patches will simplify the asm-generic implementation and remove the now
unnecessary ifdeferry.

There should be no functional change as a result of this patch.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Stafford Horne <shorne@gmail.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Jonas Bonn <jonas@southpole.se>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stefan Kristiansson <stefan.kristiansson@saunalahti.fi>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20210525140232.53872-6-mark.rutland@arm.com