openrisc: add optimized atomic operations

Using the l.lwa and l.swa atomic instruction pair.
Most openrisc processor cores provide these instructions now. If the
instructions are not available emulation is provided.

Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Stefan Kristiansson <stefan.kristiansson@saunalahti.fi>
[shorne@gmail.com: remove OPENRISC_HAVE_INST_LWA_SWA config suggesed by
Alan Cox https://lkml.org/lkml/2014/7/23/666]
[shorne@gmail.com: expand to implement all ops suggested by Peter
Zijlstra https://lkml.org/lkml/2017/2/20/317

]
Signed-off-by: Stafford Horne <shorne@gmail.com>