timekeeping: Expose API allowing retrival of current clocksource and counter value

A number of PTP drivers (such as ptp-kvm) are assuming what the
current clock source is, which could lead to interesting effects on
systems where the clocksource can change depending on external events.

For this purpose, add a new API that retrives both the current
monotonic clock as well as its counter value.

Signed-off-by: Marc Zyngier <maz@kernel.org>