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: NAndre Przywara <andre.przywara@arm.com>
Signed-off-by: NJianyong Wu <jianyong.wu@arm.com>
Signed-off-by: NMarc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20201209060932.212364-6-jianyong.wu@arm.com