arm64: arch_timer: Move clocksource_counter and co around

In order to access clocksource_counter from the errata handling code,
move it (together with the related structures and functions) towards
the top of the file.

Acked-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>