OMAP3: PM: PRCM context save/restore

Add context save and restore for PRCM module to support off-mode.
Additional registers (CM_CLKSEL4, CM_CLKEN, CM_CLKEN2) added by Tero
Kristo.

Missing CM_CLKEN_PLL_IVA2 register added by Kalle Jokiniemi.

Signed-off-by: Rajendra Nayak <rnayak@ti.com>
Signed-off-by: Tero Kristo <tero.kristo@nokia.com>
Signed-off-by: Kalle Jokiniemi <kalle.jokiniemi@digia.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>