pcie: enable Extended tag field support

>From what I read PCI has 32 transactions, PCI Express devices can handle
256 with Extended tag enabled (spec mentions also larger values but I
lack PCIe knowledge).

QEMU leaves 'Extended tag field' with 0 as value:

Capabilities: [e0] Express (v1) Root Complex Integrated Endpoint, IntMsgNum 0
        DevCap: MaxPayload 128 bytes, PhantFunc 0
                ExtTag- RBE+ FLReset- TEE-IO-

SBSA ACS has test 824 which checks for PCIe device capabilities. BSA
specification [1] (SBSA is on top of BSA) in section F.3.2 lists
expected values for Device Capabilities Register:

Device Capabilities Register     Requirement
Role based error reporting       RCEC and RCiEP: Hardwired to 1
Endpoint L0s acceptable latency  RCEC and RCiEP: Hardwired to 0
L1 acceptable latency            RCEC and RCiEP: Hardwired to 0
Captured slot power limit scale  RCEC and RCiEP: Hardwired to 0
Captured slot power limit value  RCEC and RCiEP: Hardwired to 0
Max payload size                 value must be compliant with PCIe spec
Phantom functions                RCEC and RCiEP: Recommendation is to
                                 hardwire this bit to 0.
Extended tag field               Hardwired to 1

1. https://developer.arm.com/documentation/den0094/c/

This change enables Extended tag field. All versioned platforms should
have it disabled for older versions (tested with Arm/virt).

Signed-off-by: Marcin Juszkiewicz <marcin.juszkiewicz@linaro.org>
Message-Id: <20241023113820.486017-1-marcin.juszkiewicz@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

include/hw/pci/pci.h

@@ -214,6 +214,8 @@ enum {
     QEMU_PCIE_ERR_UNC_MASK = (1 << QEMU_PCIE_ERR_UNC_MASK_BITNR),
 #define QEMU_PCIE_ARI_NEXTFN_1_BITNR 12
     QEMU_PCIE_ARI_NEXTFN_1 = (1 << QEMU_PCIE_ARI_NEXTFN_1_BITNR),
+#define QEMU_PCIE_EXT_TAG_BITNR 13
+    QEMU_PCIE_EXT_TAG = (1 << QEMU_PCIE_EXT_TAG_BITNR),
 };
 
 typedef struct PCIINTxRoute {