Documentation: PCI: endpoint/pci-endpoint-cfs: Guide to use SR-IOV

Add Documentation to help users use PCI endpoint to create virtual
functions using configfs. An endpoint function is designated as a
virtual endpoint function device when it is linked to a physical
endpoint function device (instead of a endpoint controller).

Link: https://lore.kernel.org/r/20210819123343.1951-9-kishon@ti.com
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>

Documentation/PCI/endpoint/pci-endpoint-cfs.rst

@@ -43,6 +43,7 @@ entries corresponding to EPF driver will be created by the EPF core.
 		.. <EPF Driver1>/
 			... <EPF Device 11>/
 			... <EPF Device 21>/
+			... <EPF Device 31>/
 		.. <EPF Driver2>/
 			... <EPF Device 12>/
 			... <EPF Device 22>/
@@ -68,6 +69,7 @@ created)
 				... subsys_vendor_id
 				... subsys_id
 				... interrupt_pin
+			        ... <Symlink EPF Device 31>/
                                 ... primary/
 			                ... <Symlink EPC Device1>/
                                 ... secondary/
@@ -79,6 +81,13 @@ interface should be added in 'primary' directory and symlink of endpoint
 controller connected to secondary interface should be added in 'secondary'
 directory.
 
+The <EPF Device> directory can have a list of symbolic links
+(<Symlink EPF Device 31>) to other <EPF Device>. These symbolic links should
+be created by the user to represent the virtual functions that are bound to
+the physical function. In the above directory structure <EPF Device 11> is a
+physical function and <EPF Device 31> is a virtual function. An EPF device once
+it's linked to another EPF device, cannot be linked to a EPC device.
+
 EPC Device
 ==========
 
@@ -98,7 +107,8 @@ entries corresponding to EPC device will be created by the EPC core.
 
 The <EPC Device> directory will have a list of symbolic links to
 <EPF Device>. These symbolic links should be created by the user to
-represent the functions present in the endpoint device.
+represent the functions present in the endpoint device. Only <EPF Device>
+that represents a physical function can be linked to a EPC device.
 
 The <EPC Device> directory will also have a *start* field. Once
 "1" is written to this field, the endpoint device will be ready to