x86: qemu: Enable legacy IDE I/O ports decode

QEMU always decode legacy IDE I/O ports on PIIX chipset. However Linux ata_piix
driver does sanity check to see whether legacy ports decode is turned on.
To make Linux ata_piix driver happy, turn on the decode via IDE_TIMING register.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>