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>
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>