drivers: sam: entropy: implement get_entropy_isr

The commit contributes the implementation of
get_entropy_isr API function for the SAM entropy
driver. The implementation is similar to get_entropy,
with the difference that it does not invoke k_yield()
when called with the ENTROPY_BUSYWAIT options flag set.
When the function is invoked without the ENTROPY_BUSYWAIT
flag, it simply returns whatever data is available,
without busy waiting on the RNG herdware.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>