drivers: flash: stm32g0: preparation for dual bank handling

This commit makes no functional changes, it only refactors the
driver such that dual bank flash handling can be easily added.

Instead of using HAL macros directly in the code, new macros
with STM32G0 prefix are defined.
The erase_page function gets passed the offset instead of the page,
and the FLASH CR reg is written once with all erase parameters.
flash_stm32_wait_flash_idle is already called before each
write to CR, consequently it is also made sure CFGBSY flag
is not set.

Signed-off-by: Thomas Stranger <thomas.stranger@outlook.com>