ARM: OMAP3: CM/control: move CM scratchpad save to CM driver

OMAP3 PM code for off-mode currently saves the scratchpad contents for CM
registers within OMAP control module driver. However, as we are separating
CM code into its own driver, this must be moved also. This patch adds a
new API for saving the CM scratchpad contents and uses this from the high
level scratchpad save function.

Signed-off-by: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Paul Walmsley <paul@pwsan.com>