nds32: Build infrastructure

This patch adds Makefile, Kconfig and vmlinux.lds.S files required for building
an nds32 kernel.

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