cpufeature: avoid warning when compiling with clang

The table id (second) argument to MODULE_DEVICE_TABLE is often
referenced otherwise. This is not the case for CPU features. This
leads to warnings when building the kernel with Clang:
  arch/arm/crypto/aes-ce-glue.c:450:1: warning: variable
    'cpu_feature_match_AES' is not needed and will not be emitted
    [-Wunneeded-internal-declaration]
  module_cpu_feature_match(AES, aes_init);
  ^

Avoid warnings by using __maybe_unused, similar to commit 1f318a8b
("modules: mark __inittest/__exittest as __maybe_unused").

Fixes: 67bad2fd ("cpu: add generic support for CPU feature based module autoloading")
Signed-off-by: Stefan Agner <stefan@agner.ch>
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>