kernel: add calibration_delay_done()

Add calibration_delay_done() call and dummy implementation. This allows
architectures to stop accepting registrations for new timer based delay
functions.

Signed-off-by: Peter De Schrijver <pdeschrijver@nvidia.com>
Acked-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Stephen Warren <swarren@nvidia.com>