ARM: mxs: constify platform_suspend_ops

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: Shawn Guo <shawnguo@kernel.org>