parisc: Show MPE/iX model string at bootup

Some (mostly 64-bit machines) machines allow to run MPE/iX and report the MPE
model string via firmware call. Enhance the pdc_model_sysmodel() function to
report that model string.
Note that some 32-bit machines like the B160L wrongly report success for the
firmware call, so include a check to prevent showing wrong info.

Signed-off-by: Helge Deller <deller@gmx.de>