Add support for STM32G0 chips

[WRansohoff]

It'd be nice to support the fairly new STM32G0 line of chips.

• Programmer/board type: Stlink/v2
• Programmer firmware version: V2.J29.S7
• Operating system: Ubuntu 18.04 (all platforms?)
• Stlink tools version and/or git commit hash: git-7651d2116fd74c7803ea00ab1da7cf3d00faf44c
• Stlink commandline tool name: st-util, st-flash
• Target chip (and optional board): STM32G071GB

Right now they aren't supported:

2019-01-12T16:04:23 INFO common.c: Loading device parameters....
2019-01-12T16:04:23 WARN common.c: unknown chip id! 0x20006460

And it's possible that I'm reading the manuals wrong, but it also looks like the core flash register offsets (CR, SR, KEYR) are slightly different from most other STM32 lines.

I wrote a small patch that seems to work for erasing and flashing an STM32G071GB chip, but I have only tested it with a small LED-blinking example which only occupies a single page of Flash:

WRansohoff@c1ad2cf

And besides the lack of testing, I don't know how well that conforms to the rest of the project's code style and formatting, so it's probably not worth making a pull request yet.

[xor-gate]

Hi @WRansohoff, thanks for improving stlink. I had a look at your code and it looks fine. You may provide a PR!

[WRansohoff]

Sure, here - I do want to clean it up a bit, but it's something to start with:

#760

[yaqwsx]

I am trying to use st-flash with STM32G071GB. It works well for small programs, however, once the program size is larger than 4096 bytes, it fails to verify the program (--debug log here: https://pastebin.com/5dtTYqp9):

st-flash 1.5.1-24-gfeb8661
2019-04-25T15:49:58 INFO common.c: Loading device parameters....
2019-04-25T15:49:58 INFO common.c: Device connected is: G071/G081 device, id 0x20006460
2019-04-25T15:49:58 INFO common.c: SRAM size: 0x9000 bytes (36 KiB), Flash: 0x20000 bytes (128 KiB) in pages of 2048 bytes
2019-04-25T15:49:58 INFO common.c: Attempting to write 5040 (0x13b0) bytes to stm32 address: 134217728 (0x8000000)
Flash page at addr: 0x08001000 erased
2019-04-25T15:49:58 INFO common.c: Finished erasing 3 pages of 2048 (0x800) bytes
Writing
Starting   2 page write
2019-04-25T15:49:58 INFO common.c: Starting verification of write complete
2019-04-25T15:49:59 ERROR common.c: Verification of flash failed at offset: 4096
stlink_fwrite_flash() == -1

Is it possible there is a bug in the patch introduced by @WRansohoff?