nds32: Process management

This patch includes copy_thread(), start_thread() implementation and cpu_context
structure definition. nds32 uses $r25 to get current task_struct.

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