clockchips: Add FEAT_PERCPU clockevent flag

Add the flag CLOCK_EVT_FEAT_PERCPU which is supposed to be set for per
cpu clockevent devices.

Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Michal Simek <michal.simek@xilinx.com>