microblaze: Use asm generic cmpxchg.h for !SMP case

The whole implementation is done in asm-generic/cmpxchg.h file and there is
no reason to duplicate it.
Also do not include asm-generic/cmpxchg-local.h because it is already
included from asm-generic/cmpxchg.h

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Stefan Asserhall <stefan.asserhall@xilinx.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>