drivers: flash: stm32l4x: Support SOCs with < 1MB

Up to now, the only SOC that was supported in the STM32L496 series was
STM32L496XG with flash of 1MB. With the recent support of STM32L496XE,
this implementation is not correct. This patch adds support for the
other SOCs that come with flashes of 512KB and 256KB.

Signed-off-by: Ioannis Konstantelias <ikonstadel@gmail.com>