ARM: OMAP2+: CM: make some pointers and function arguments as const

Make the pointer cm_ll_data of type cm_ll_data as const as it does not
modify the fields of the structure it points too.
After this change, make the argument of cm_register function as const as
it is used to initialise cm_ll_data or used inside an if condition.
Make the pointer argument of cm_unregister function as const as it is only
used inside an if condition.
Add const to the function prototypes too.

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>