ion: use iounmap instead of __arm_iounmap

The API iounmap() is more generic and portable
than __arm_iounmap().

Change-Id: If9601e9266bec56442686d3f2bdb1d475cd7bead
Signed-off-by: Larry Bassel <lbassel@codeaurora.org>

Conflicts:
	drivers/gpu/ion/ion_cp_heap.c