Improve error messages when pdf builds fails

If PDF docs build failed, suggest user to read Prerequisites section of
documentation or use option --no-pdf.

Signed-off-by: Anton Kolesov <anton.kolesov@synopsys.com>
(cherry picked from commit 4d0c8ec3a28e1867b136b5bb1f2dd78ef8764999)
Add EM AUX registers XML file for Opella-XD

Add a file that contains XML description of AUX registers of EM core. This
file is required by the Ashling gdbserver for the Opella-XD probe.

Signed-off-by: Anton Kolesov <anton.kolesov@synopsys.com>
(cherry picked from commit f1bf01f07ba69c9f9860b488c2fc800a10c85e93)