arm64: add credited/trusted RNG support

arm64: add credited/trusted RNG support

Currently arm64 doesn't initialize the primary CRNG in a (potentially)
trusted manner as we only detect the presence of the RNG once secondary
CPUs are up.

Now that the core RNG code distinguishes the early initialization of the
primary CRNG, we can implement arch_get_random_seed_long_early() to
support this.

This patch does so.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: Theodore Ts'o <tytso@mit.edu>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20200210130015.17664-4-mark.rutland@arm.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>