ARM: mm: Remove unused arm_vmregion priv field

Commit e9da6e99

 ("ARM: dma-mapping:
remove custom consistent dma region") removed the last users of the
field. Remove it.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>