[STM32H735]: Set hardware breakpoints for external bus

[c-grant]

Programmer/board type: [STLINK V3 on STM32H735-DK]
Operating system an version: [Windows 10 19042.1083]
stlink tools version and/or git commit hash: [1.7.0] + pull requests to access external memory
stlink commandline tool name: [st-util]
Target chip (and board, if applicable): [STM32H735 on STM32H735-DK]

GDB is setting breakpoints based on the XML memory map designation of "rom" or "ram", which is hardcoded in st-util for a given processor

However the external bus can be RAM or ROM depending on design. The STM32H735-DK has external FLASH at address 0x90000000. As a result, because the entire external memory range is "ram" as it could be either, software breakpoints (Z0) get sent when a breakpoint is created and the never get tripped as the memory area is read only.