arm64: mark cpus_have_hwcap as __maybe_unused

cpus_have_hwcap() is defined as a 'static' function an only used in
one place that is inside of an #ifdef, so we get a warning when
the only user is disabled:

arch/arm64/kernel/cpufeature.c:699:13: warning: 'cpus_have_hwcap' defined but not used [-Wunused-function]

This marks the function as __maybe_unused, so the compiler knows that
it can drop the function definition without warning about it.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 37b01d53 ("arm64/HWCAP: Use system wide safe values")
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>