nds32/ag101: cpu and init funcs of SoC ag101

SoC ag101 is the first chip using NDS32 N1213 cpu core.
Add header file of device offset support for SoC ag101.
Add main function of SoC ag101 based on NDS32 n1213 core.
Add lowlevel_init.S and other periphal related code.

This version of lowlevel_init.S also replace hardcode value
by MARCO defines from the GPL version andesboot for better
code quality.

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