nios2: remove wrapper header for cmpxchg.h

Since commit 713e9b80 ("nios2: Switch to generic __xchg()")
asm/cmpxchg.h for nios2 is merely including asm-generic/cmpxchg.h. Thus,
the wrapper can be omitted and the generic header can be used directly.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Acked-by: Ley Foon Tan <ley.foon.tan@intel.com>