Documentation: PCI: Add binding documentation for pci-test endpoint function

Add binding documentation for pci-test endpoint function that helps in
adding and configuring pci-test endpoint function.

Signed-off-by: Kishon Vijay Abraham I <[e-mail]>
Signed-off-by: Bjorn Helgaas <[e-mail]>

Documentation/PCI/00-INDEX

···
 	- guide to use configfs to configure the PCI endpoint function.
 endpoint/pci-test-function.txt
 	- specification of *PCI test* function device.
+endpoint/function/binding/
+	- binding documentation for PCI endpoint function

Documentation/PCI/endpoint/function/binding/pci-test.txt

···
+PCI TEST ENDPOINT FUNCTION
+
+name: Should be "pci_epf_test" to bind to the pci_epf_test driver.
+
+Configurable Fields:
+vendorid	 : should be 0x104c
+deviceid	 : should be 0xb500 for DRA74x and 0xb501 for DRA72x
+revid		 : don't care
+progif_code	 : don't care
+subclass_code	 : don't care
+baseclass_code	 : should be 0xff
+cache_line_size	 : don't care
+subsys_vendor_id : don't care
+subsys_id	 : don't care
+interrupt_pin	 : Should be 1 - INTA, 2 - INTB, 3 - INTC, 4 -INTD
+msi_interrupts	 : Should be 1 to 32 depending on the number of MSI interrupts
+		   to test