Documentation: PCI: Add configfs binding documentation for pci-ntb endpoint function

.. SPDX-License-Identifier: GPL-2.0

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PCI NTB Endpoint Function

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1) Create a subdirectory to pci_epf_ntb directory in configfs.

Standard EPF Configurable Fields:

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vendorid	   should be 0x104c

deviceid	   should be 0xb00d for TI's J721E SoC

revid		   don't care

progif_code	   don't care

subclass_code	   should be 0x00

baseclass_code	   should be 0x5

cache_line_size	   don't care

subsys_vendor_id   don't care

subsys_id	   don't care

interrupt_pin	   don't care

msi_interrupts	   don't care

msix_interrupts	   don't care

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2) Create a subdirectory to directory created in 1

NTB EPF specific configurable fields:

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db_count	   Number of doorbells; default = 4

mw1     	   size of memory window1

mw2     	   size of memory window2

mw3     	   size of memory window3

mw4     	   size of memory window4

num_mws     	   Number of memory windows; max = 4

spad_count     	   Number of scratchpad registers; default = 64

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   pci-ntb-function

   function/binding/pci-test

   function/binding/pci-ntb