clocksource: Add clocksource id for arm arch counter

Add clocksource id to the ARM generic counter so that it can be easily
identified from callers such as ptp_kvm.

Cc: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Jianyong Wu <jianyong.wu@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20201209060932.212364-6-jianyong.wu@arm.com