powerpc/pseries: Implement arch_get_random_long() based on H_RANDOM

Add support for the arch_get_random_long() hook based on the H_RANDOM
hypervisor call. We trust the hypervisor to provide us with random data,
ie. we don't whiten it in anyway.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>