nfit: Mark some functions as __maybe_unused

On arm64 little endian allyesconfig:

drivers/acpi/nfit/intel.c:149:12: warning: unused function 'intel_security_unlock' [-Wunused-function]
static int intel_security_unlock(struct nvdimm *nvdimm,
           ^
drivers/acpi/nfit/intel.c:230:12: warning: unused function 'intel_security_erase' [-Wunused-function]
static int intel_security_erase(struct nvdimm *nvdimm,
           ^
drivers/acpi/nfit/intel.c:279:12: warning: unused function 'intel_security_query_overwrite' [-Wunused-function]
static int intel_security_query_overwrite(struct nvdimm *nvdimm)
           ^
drivers/acpi/nfit/intel.c:316:12: warning: unused function 'intel_security_overwrite' [-Wunused-function]
static int intel_security_overwrite(struct nvdimm *nvdimm,
           ^
4 warnings generated.

Mark these functions as __maybe_unused because they are only used when
CONFIG_X86 is set.

Fixes: 4c6926a2 ("acpi/nfit, libnvdimm: Add unlock of nvdimm support for Intel DIMMs")
Suggested-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>