hwrng: add sandbox driver interface to host /dev/random ...

Linux as well as other operating systems can provide /dev/random and
/dev/urandom device to service userspace need for randomness.
Add a driver to use /dev/random for blocking and /dev/urandom for
non-blocking barebox random numbers.

Signed-off-by: Ahmad Fatoum <ahmad@a3f.at>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>