arm64: arch_timer: mark functions as __always_inline

If CONFIG_FUNCTION_GRAPH_TRACER is enabled function
arch_counter_get_cntvct() is marked as notrace. However, function
__arch_counter_get_cntvct is marked as inline. If
CONFIG_OPTIMIZE_INLINING is set that will make the two functions
tracable which they shouldn't.

Rework so that functions __arch_counter_get_* are marked with
__always_inline so they will be inlined even if CONFIG_OPTIMIZE_INLINING
is turned on.

Fixes: 0ea41539 ("clocksource/arm_arch_timer: Use arch_timer_read_counter to access stable counters")
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>