drivers: flash: stm32: Update flash driver implementation

Remove soc/arm/st_stm32/stm32YY/flash_registers.h files.
Change register accesses in stm32 flash drivers to use FLASH_TypeDef
from modules/hal/stm32/stm32cube/stm32YYxx/soc/stm32xxxxxx.h.

Fixes #16235

Signed-off-by: Sarvesh Patkar <psarvesh314@gmail.com>