xlat_tables_v2: fix signed/unsigned comparisons

This patch changes input param level in xlat_tables_print_internal() to
an unsigned int to fix the signed/unsigned comparison warnings. The
compiler complains about these warnings, thus halting the build flow
for Tegra platforms.

Change-Id: Ieccc262a63daca7a26ca6a14d81466397af8b89f
Signed-off-by: Varun Wadekar <vwadekar@nvidia.com>
Signed-off-by: David Cunado <david.cunado@arm.com>