arm64: random: implement arch_get_random_int/_long based on RNDR

When support for RNDR/RNDRRS was introduced, we elected to only
implement arch_get_random_seed_int/_long(), and back them by RNDR
instead of RNDRRS. This was needed to prevent potential performance
and/or starvation issues resulting from the fact that the /dev/random
driver used to invoke these routines on various hot paths.

These issues have all been addressed now [0] [1], and so we can wire up
this API more straight-forwardly:

- map arch_get_random_int/_long() onto RNDR, which returns the output of
  a DRBG that is reseeded at an implemented defined rate;
- map arch_get_random_seed_int/_long() onto the TRNG firmware service,
  which returns true, conditioned entropy, or onto RNDRRS if the TRNG
  service is unavailable, which returns the output of a DRBG that is
  reseeded every time it is used.

[0] 390596c9959c random: avoid arch_get_random_seed_long() when collecting IRQ randomness
[1] 2ee25b6968b1 random: avoid superfluous call to RDRAND in CRNG extraction

Cc: Andre Przywara <[email protected]>
Cc: Mark Brown <[email protected]>
Signed-off-by: Ard Biesheuvel <[email protected]>
Acked-by: Jason A. Donenfeld <[email protected]>
Reviewed-by: Andre Przywara <[email protected]>
Reviewed-by: Mark Brown <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Will Deacon <[email protected]>

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