nds32/core N1213: NDS32 N12 core family N1213

Add N1213 cpu core (N12 Core family) support for NDS32 arch.
This patch includes start.S for the initialize procedure of N1213.

Start procedure:
 start.S will start up the N1213 CPU core at first,
 then jump to SoC dependent "lowlevel_init.S" and
 "watchdog.S" to configure peripheral devices.

Signed-off-by: Macpaul Lin <macpaul@andestech.com>
Signed-off-by: Greentime Hu <greentime@andestech.com>