Documentation: crypto: Add DT binding info for the img hw hash accelerator

This adds the binding documentation for the Imagination Technologies hash
accelerator that provides hardware acceleration for SHA1/SHA224/SHA256/MD5
hashes.  This hardware will be present in the upcoming pistachio SoC.

Signed-off-by: James Hartley <james.hartley@imgtec.com>
Reviewed-by: Andrew Bresticker <abrestic@chromium.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>