sun4u: implement power device

This inbuilt device contains a single 4-byte register, of which bit 24 is used
to power down the machine on a real Ultra 5.

The power device exists at offset 0x724000 on a real machine, but due to the
current configuration of the BARs in QEMU it must be located lower in PCI IO
space.

For the moment we place the power device at offset 0x7240 as a reminder of its
original location and raise the base PCI IO address from 0x4000 to 0x8000.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Artyom Tarasenko <atar4qemu@gmail.com>