drivers: entropy: add get_entropy_isr() for CC13x2/CC26x2

A non-blocking, isr-safe version of get_entropy() is necessary in order
to be called during boot time before POST_KERNEL initialization.
Otherwise a crash is seen as the existing get_entropy() implementation
uses k_sem and relies on interrupts.

Fixes #18629

Signed-off-by: Vincent Wan <vincent.wan@linaro.org>