crypto: img-hash - Fix img_hash_match unused warning

Mark the of_device_id table as maybe_unused.  This fixes a W=1 warning:

  drivers/crypto/img-hash.c:930:34: error: ‘img_hash_match’ defined but not used [-Werror=unused-const-variable=]

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>