crypto: update some Arm documentation references

The Arm documentation has moved to Documentation/arch/arm; update a
set of references under crypto/allwinner to match.

Cc: Corentin Labbe <clabbe.montjoie@gmail.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Chen-Yu Tsai <wens@csie.org>
Cc: Samuel Holland <samuel@sholland.org>
Acked-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>