drivers/entropy: stm32 add seed error recovery

This commit introduces an automatic recovery procedure in cases an
entropy source error was detected.
- On Series with soft reset support a soft rest is executed.
- On Series w/o soft reset support the pipeline is cleared by reading
  the RNG_DR 12 times.

With this changes the check for seed errors uses SEIS flag instead
of the SECS flag.

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