PCI: endpoint: Allow user to create sub-directory of 'EPF Device' directory

Documentation/PCI/endpoint/pci-endpoint-cfs.rst explains how a user has to
create a directory in-order to create a 'EPF Device' that can be
configured/probed by 'EPF Driver'.

Allow user to create a sub-directory of 'EPF Device' directory for any
function specific attributes that has to be exposed to the user.

Link: https://lore.kernel.org/r/20210201195809.7342-11-kishon@ti.com


Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>