pciutils: Add decode support for Data Object Exchange Extended Capability

pciutils: Add decode support for Data Object Exchange Extended Capability

PCI Data Object Exchange [1] provides a mailbox interface used as the
transport for various protocols defined by PCI-SIG and others. Make the
limited information in config space available. Note the Read/Write
Mailbox registers themselves are not currently parsed as the usefulness
of accessing one dword of a protocol is probably limited.

In future, operating systems may provide means to safely query the
supported protocols, but those have not yet been defined.

Example output:

	Capabilities: [100 v1] Data Object Exchange
		DOECap: IntSup+
			Interrupt Message Number 001
		DOECtl: IntEn+
		DOESta: Busy- IntSta- Error- ObjectReady+

[1] PCIe r6.0, sections 6.30 and 7.9.24

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>