drivers: random: Add ESP32 random number generator driver

The random number generator from ESP32 uses noise from Wi-Fi and
Bluetooth radios.  If these are off, a pseudo-random number is
generated instead; this is currently the case, but even though it's a
black box, it's arguably better than returning a timestamp as a
pseudo-random number generator.

According to the ESP32 Technical Reference manual, the RNG passed the
Dieharder Random Number Test suite (version 3.31.1)[1], but nothing has
been said about the quality of the PRNG.

The RNG register is read directly; no effort is made to use its
contents to feed an entropy pool in a way that's similar to /dev/random
on POSIX systems, as no such subsystem exists on Zephyr at the moment.

[1] http://webhome.phy.duke.edu/~rgb/General/dieharder.php



Signed-off-by: Leandro Pereira <leandro.pereira@intel.com>