mini51: support for Nuvoton NuMicro M051 series flash memory

adds flash support for Nuvoton M052, M054, M058, M0516 microcontrollers
into the mini51 driver, patch also adds support for programing LDROM,
flash data and flash config.

I've tested it on a M0516LBN microcontroller using an ST-LINK/V2:
1. removing security lock:
   openocd -f interface/stlink-v2.cfg  -f target/m051.cfg -c "init ; halt ; mini51 chip_erase; exit"
2. flashing:
   openocd -f interface/stlink-v2.cfg  -f target/m051.cfg -c "program file.hex"

Change-Id: I918bfbb42461279c216fb9c22272d77501a2f202
Signed-off-by: Pawel Si <stawel+openocd@gmail.com>
README update

Change-Id: I74edb474667c18c2b26a1c894d4288ce9e2edfa3