clocksource: add common of_clksrc_init() function

It is desirable to move all clocksource drivers to drivers/clocksource,
yet each requires its own initialization function. We'd rather not
pollute <linux/> with a header for each function. Instead, create a
single of_clksrc_init() function which will determine which clocksource
driver to initialize based on device tree.

Based on a similar patch for drivers/irqchip by Thomas Petazzoni.

Signed-off-by: Stephen Warren <swarren@nvidia.com>