hw/pci-host/astro: Add missing astro & elroy registers for NetBSD

NetBSD accesses some astro and elroy registers which aren't accessed
by Linux yet. Add emulation for those registers to allow NetBSD to
boot further.
Please note that this patch is not sufficient to completely boot up
NetBSD on the 64-bit C3700 machine yet.

Signed-off-by: Helge Deller <deller@gmx.de>
Tested-by: Bruno Haible <bruno@clisp.org>