nds32: Atomic operations

This patch includes the atomic and futex operations. Many atomic operations use
the load-lock word(llw) and store-condition word(scw) operations.

Signed-off-by: Vincent Chen <vincentc@andestech.com>
Signed-off-by: Greentime Hu <greentime@andestech.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>