arm64/arch_timer: Mark errata handlers as __maybe_unused

arm64/arch_timer: Mark errata handlers as __maybe_unused

In some rare randconfig builds, we end up with two functions being entirely
unused:

drivers/clocksource/arm_arch_timer.c:342:12: error: 'erratum_set_next_event_tval_phys' defined but not used [-Werror=unused-function]
 static int erratum_set_next_event_tval_phys(unsigned long evt,
drivers/clocksource/arm_arch_timer.c:335:12: error: 'erratum_set_next_event_tval_virt' defined but not used [-Werror=unused-function]
 static int erratum_set_next_event_tval_virt(unsigned long evt,

We could add an #ifdef around them, but we would already have to check for
several symbols there and there is a chance this would get more complicated
over time, so marking them as __maybe_unused is the simplest way to avoid the
harmless warnings.

Fixes: 01d3e3ff2608 ("arm64: arch_timer: Rework the set_next_event workarounds")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: Christoffer Dall <christoffer.dall@linaro.org>
Link: http://lkml.kernel.org/r/20170419173737.3846098-1-arnd@arndb.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>