hw/hppa: Move software power button address back into PDC

The various operating systems (e.g. Linux, NetBSD) have issues
mapping the power button when it's stored in page zero.
NetBSD even crashes, because it fails to map that page and then
accesses unmapped memory.

Since we now have a consistent memory mapping of PDC in 32-bit
and 64-bit address space (the lower 32-bits of the address are in
sync) the power button can be moved back to PDC space.

This patch fixes the power button on Linux, NetBSD and HP-UX.

Signed-off-by: Helge Deller <deller@gmx.de>
Tested-by: Bruno Haible <bruno@clisp.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
(cherry picked from commit ed35afcb331a972210816435d6b1b5de17fc7d4f)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>