crypto: arm64 - Fix unused variable compilation warnings of cpu_feature

The cpu feature defined by MODULE_DEVICE_TABLE is only referenced when
compiling as a module, and the warning of unused variable will be
encountered when compiling with intree. The warning can be removed by
adding the __maybe_unused flag.

Fixes: 03c9a333 ("crypto: arm64/ghash - add NEON accelerated fallback for 64-bit PMULL")
Fixes: ae1b83c7 ("crypto: arm64/sm4 - add CE implementation for GCM mode")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Tianjia Zhang <tianjia.zhang@linux.alibaba.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>