nds32: Kernel booting and initialization

This patch includes the kernel startup code. It can get dtb pointer
passed from bootloader. It will create a temp mapping by tlb
instructions at beginning and goto start_kernel.

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