drivers: flash: stm32: mv security-mode dependent defines to header

An attempt to simplify the ifdef-ery around FLASH_SR is made.
Define Registers and flags in the header file instead of including
several individual operations in the driver.

FLASH_FLAG_BSY is not only defined for STM32L5, but also other series.
Therefore use this flag instead of FLASH_SR_BSY.
Only the g0 series definition is not valid in our context,
therefore use FLASH_SR_BSY1 instead.

No functional changes, only refactoring.

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