This is a prep work commit for adding flash page layout support.
The internal flash APIs for STM32 on L4 and F4 are a bit
inconsistent. Some routines take a pointer to the private data, others
don't. Those that do take it as a last argument instead of a
first. Additionally, some APIs are declared in flash_stm32.h that
aren't ever used by the family-specific files.
Clean this up by making everything take a struct device*, and marking
routines static in the common driver code when possible.
Signed-off-by: Marti Bolivar <marti.bolivar@linaro.org>