clk: ti: move OMAP4+ DPLL implementation under drivers/clk

With the legacy clock support gone, the OMAP4 specific DPLL implementations
can be moved under the clock driver. Change some of the function prototypes
to be static at the same time, and remove some exports from the global TI
clock driver header.

Signed-off-by: Tero Kristo <t-kristo@ti.com>