Add nds32 ag101p generic board support.

Signed-off-by: Kun-Hua Huang <kunhua@andestech.com>

board:
Add config file of board adp-ag102
Add adp-ag102 into boards.cfg
Add adp-ag102 into MAINTAINERS

doc:
add README of ag102

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