Freescale ION unmapped heap for i.MX

Unmapped heap are used for security purpose, providing secure data
path buffers for video decode and display.

Required properties
- compatible:
  -  "fsl,display-heap" for display ion heap
  -  "fsl,vpu-heap" for vpu decoder ion heap

Required properties for Ion heaps
- memory-region: one reserved mem region pointer.

Example:
  ion-display-region {
    compatible = "fsl,display-heap", "linux,ion-heap-unmapped";
    memory-region = <&display_region>;
  };