drivers: flash: stm32: improve timeout handling

The timeout handling in the STM32 flash driver is currently a counter
in the busy check loop. This doesn't scale well across the whole STM32
family:
- The duration of the loop depends on the CPU speed, the activation of
  caches.
- The duration of the longest flash operation (a sector erase as the
  driver doesn't support mass erase) can varies a lot mostly depending
  on the sector size, 2K on F0 and L4 families or 128K on the F4 family.

In addition the timeout can change depending if the writing thread is
prempted or not.

Fix that by defining a timeout in ms depending on the family, and using
k_uptime_get to get a precise measurement of the time.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>