It will be used by random_kthread from Yarrow as a hardware source of entropy
whenever the thread wakes up, which happens around 10 times a second.

Systems that don't provide a hardware for rng will lack the property, then
should gather environmental noise from specific interrupts.
Systems that do provide will still gather environmental noise from interrupts
if available, and do 'one round' of gathering from the rng hardware.

Signed-off-by: Raphael S. Carvalho <raphaelsc@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>