flash/nor: flash driver for Synwit SWM050 MCUs

flash/nor: flash driver for Synwit SWM050 MCUs

SWM050 is a series of MCU product by Foshan Synwit Tech, which is
available in TSSOP-8 or SSOP-16 packages.

Adds flash driver for the internal 8KiB flash of the MCU. The registers
are based on reverse engineering the J-Flash blob provided by the
vendor.

Also adds a pre-made cfg file.

Change-Id: I0b29f0c0d062883542ee743e0750a4c6b6609ebd
Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Signed-off-by: Caleb Szalacinski <contact@skiboy.net>
Reviewed-on: http://openocd.zylin.com/4927
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>