clocksource: always define CLOCKSOURCE_OF_DECLARE

This allows clocksource drivers that support both DT and non-DT to
always invoke macro CLOCKSOURCE_OF_DECLARE(), rather than wrapping
it in a #ifdef CONFIG_CLKSRC_OF, which simplifies their code.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Olof Johansson <olof@lixom.net>