arm: omap2+ remove dead clock code

Remove omap_clocks_register and dummy_ck. The former is not used anymore
now that the statically defined clk stuctures are replaced with proper
descriptors and registered with the framework.

The dummy clock in arch/arm/mach-omap2 is made redundant by the OMAP3+
clock data that migrated to drivers/clk.

An additional benefit to this clean-up is removing the references to
clk-private.h which will be removed.

Cc: Paul Walmsley <paul@pwsan.com>
Cc: Tero Kristo <t-kristo@ti.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Michael Turquette <mturquette@linaro.org>