flash/stm32l4x: add support of STM32G0Bx/G0Cx devices

flash/stm32l4x: add support of STM32G0Bx/G0Cx devices

this device has a dual bank flash architecture up to 512 KB (page 2KB)
reference: RM0444 Rev 5

notes:
 - 128k variant is always single bank
 - 256k variant flash is contiguous (no gap) in dual bank mode
 - BKER is bit 13 vs bit 11 for other devices
   > added cr_bker_mask in stm32l4_flash_bank struct
 - BSY2 for bank 2 operations
   > added sr_bsy_mask in stm32l4_flash_bank struct
   > proposed optimization: always wait for (BSY1 | BSY2) with
     STM32G0Bx/G0Cx devices only (for L4+ devices BSY2=PEMPTY)

TODO: update flashloader to use the proper BSY bits
      temporarily don't use the loader in dual bank mode

Change-Id: I54b0c93b494e7209da818791d15edd8cd42c2732
Signed-off-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/6036
Reviewed-by: Oleksij Rempel <linux@rempel-privat.de>
Tested-by: jenkins