hw/arm/virt-acpi-build: _CCA attribute is compulsory

According to ACPI specification 6.2.17 _CCA (Cache Coherency Attribute)
this attribute is compulsory on ARM systems. Add this attribute to
the PCI host bridges as required.

Without this the kernel will produce the error
[Firmware Bug]: PCI device 0000:00:00.0 fail to setup DMA.

Signed-off-by: Graeme Gregory <graeme.gregory@linaro.org>
Message-id: 1446460786-13663-1-git-send-email-graeme.gregory@linaro.org
Reviewed-by: Shannon Zhao <shannon.zhao@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>