powerpc/4xx: Constify cpm_suspend_ops

struct platform_suspend_ops are not supposed to change at runtime.
Functions suspend_set_ops working with const platform_suspend_ops. So
mark the non-const structs as const.

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>