xlat_tables_v2: get unmap action type with helper function.

This patch introduces helper function 'xlat_tables_unmap_region_action'
to get the required action type from given arguments when unmapping the
specified region.
it reduces cyclomatic code complexity in xlat_tables_unmap_region function.

Cyclomatic complexity calculated using 'Coverity'

fixes arm-software/tf-issues#673

Signed-off-by: David Pu <dpu@nvidia.com>