parisc/power: Fix power soft-off when running on qemu

Firmware returns the physical address of the power switch,
so need to use gsc_writel() instead of direct memory access.

Fixes: d0c21947

 ("parisc/power: Add power soft-off when running on qemu")
Signed-off-by: Helge Deller <deller@gmx.de>
Cc: stable@vger.kernel.org # v6.0+