clk: Add functions to save/restore clock context en-masse

Deep enough power saving mode can result into losing context of the clock
registers also, and they need to be restored once coming back from the power
saving mode. Hence add functions to save/restore clock context.

Signed-off-by: Keerthy <j-keerthy@ti.com>
Signed-off-by: Russ Dill <Russ.Dill@ti.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Tero Kristo <t-kristo@ti.com>