Documentation: misc-devices: Add Documentation for pci-endpoint-test driver

Add Documentation for pci-endpoint-test driver.
Signed-off-by: NKishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: NBjorn Helgaas <bhelgaas@google.com>

Documentation/misc-devices/pci-endpoint-test.txt

+Driver for PCI Endpoint Test Function
+
+This driver should be used as a host side driver if the root complex is
+connected to a configurable PCI endpoint running *pci_epf_test* function
+driver configured according to [1].
+
+The "pci_endpoint_test" driver can be used to perform the following tests.
+
+The PCI driver for the test device performs the following tests
+	*) verifying addresses programmed in BAR
+	*) raise legacy IRQ
+	*) raise MSI IRQ
+	*) read data
+	*) write data
+	*) copy data
+
+This misc driver creates /dev/pci-endpoint-test.<num> for every
+*pci_epf_test* function connected to the root complex and "ioctls"
+should be used to perform the above tests.
+
+ioctl
+-----
+ PCITEST_BAR: Tests the BAR. The number of the BAR to be tested
+	      should be passed as argument.
+ PCITEST_LEGACY_IRQ: Tests legacy IRQ
+ PCITEST_MSI: Tests message signalled interrupts. The MSI number
+	      to be tested should be passed as argument.
+ PCITEST_WRITE: Perform write tests. The size of the buffer should be passed
+		as argument.
+ PCITEST_READ: Perform read tests. The size of the buffer should be passed
+	       as argument.
+ PCITEST_COPY: Perform read tests. The size of the buffer should be passed
+	       as argument.
+
+[1] -> Documentation/PCI/endpoint/function/binding/pci-test.txt