random: add_hwgenerator_randomness() for feeding entropy from devices

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

Change-Id: Id9031d6b615877ea957ee2d2aba1f8150ec699b9
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>
Git-commit: c84dbf61a7b322188d2a7fddc0cc6317ac6713e2
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git


[sboyd@codeaurora.org: Bring in ENTROPY_BITS() define,
s/random_write_wakeup_bits/random_write_wakeup_thresh/, and pass
NULL to mix_pool_bytes last parameter]
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>