sticon/parisc: Allow 64-bit STI calls in PDC firmware abstration

Some 64-bit machines require us to call the STI ROM in 64-bit mode, e.g.
with the VisFXe graphic card.
This patch allows drivers to use such 64-bit calling conventions.

Tested-by: John David Anglin <dave.anglin@bell.net>
Signed-off-by: Helge Deller <deller@gmx.de>