omap: IOMMU: add missing function declaration

Declaration of exported function 'iopgtable_lookup_entry' is missing from
header file. Currently we have a sparse warning as it's not being used
externally. Adding its declaration to avoid such warning and allow its usage
in future.

Signed-off-by: David Cohen <dacohen@gmail.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>