sticon/parisc: Fix STI console on 64-bit only machines

Fix the STI console to be able to execute either the 64-bit STI ROM code
or the 32-bit STI ROM code.

This is necessary on 64-bit only machines (e.g. C8000 workstation) which
otherwise won't show the STI text console with HP graphic cards like
Visualize-FX5/FX10/FXe.

Note that when calling 32-bit code from a 64-bit kernel one needs to
copy contents on the CPU stack from high memory down below the 4GB
limit.

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