clk: ti: add core support for initializing legacy clocks

Legacy clock data for OMAP3 is being moved under clock driver, thus
base support for this is needed. This patch adds basic definitions for
clock init descriptors and core infrastructure for initialization,
which will be called from the OMAP3 clock init.

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