hw/arm/aspeed: simplify using the 'unimplemented device' for aspeed_soc.io

hw/arm/aspeed: simplify using the 'unimplemented device' for aspeed_soc.io

(qemu) info mtree
 address-space: cpu-memory-0
   0000000000000000-ffffffffffffffff (prio 0, i/o): system
     0000000000000000-0000000007ffffff (prio 0, rom): aspeed.boot_rom
-    000000001e600000-000000001e7fffff (prio -1, i/o): aspeed_soc.io
+    000000001e600000-000000001e7fffff (prio -1000, i/o): aspeed_soc.io
     000000001e620000-000000001e6200ff (prio 0, i/o): aspeed.smc.ast2500-fmc
     000000001e630000-000000001e6300ff (prio 0, i/o): aspeed.smc.ast2500-spi1
     000000001e631000-000000001e6310ff (prio 0, i/o): aspeed.smc.ast2500-spi2

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Message-id: 20180209085755.30414-3-f4bug@amsat.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>