random: add_hwgenerator_randomness() for feeding entropy from devices

This patch adds an interface to the random pool for feeding entropy
in-kernel.

Signed-off-by: Torsten Duwe <duwe@suse.de>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Acked-by: H. Peter Anvin <hpa@zytor.com>