xlat_tables_v2: find VA/idx with helper functions.

This patch introduces 2 helper functions 'xlat_tables_find_start_va' and
'xlat_tables_va_to_index' to find the first VA and table index affected by the
specified mmap region. it reduces code duplication and cyclomatic code
complexity in xlat_tables_map/unmap_region functions.

Cyclomatic complexity calculated using 'Coverity'

fixes arm-software/tf-issues#673

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