clocksource/drivers/arm_arch_timer: Add __ro_after_init and __init

Some functions are not needed after booting, so mark them as __init
to move them to the .init section.

Some global variables are never modified after init, so can be
__ro_after_init.

Signed-off-by: Jisheng Zhang <Jisheng.Zhang@synaptics.com>
Acked-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20210330140444.4fb2a7cb@xhacker.debian