clocksource/arm_arch_timer: Drop use of static key in arch_timer_reg_read_stable

Let's start with the removal of the arch_timer_read_ool_enabled
static key in arch_timer_reg_read_stable. It is not a fast path,
and we can simplify things a bit.

Acked-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>