docs: ABI: Add sysfs documentation interface of dw-xdata-pcie driver

This patch describes the sysfs interface implemented on the dw-xdata-pcie
driver.
Signed-off-by: Gustavo Pimentel <gustavo.pimentel@synopsys.com>
Link: https://lore.kernel.org/r/438c4ca9f6cc9e1cb29a65c0d2cca9a3d3f181b1.1617016509.git.gustavo.pimentel@synopsys.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/ABI/testing/sysfs-driver-xdata

+What:		/sys/class/misc/drivers/dw-xdata-pcie.<device>/write
+Date:		April 2021
+KernelVersion:	5.13
+Contact:	Gustavo Pimentel <gustavo.pimentel@synopsys.com>
+Description:	Allows the user to enable the PCIe traffic generator which
+		will create write TLPs frames - from the Root Complex to the
+		Endpoint direction or to disable the PCIe traffic generator
+		in all directions.
+
+		Write y/1/on to enable, n/0/off to disable
+
+		Usage e.g.
+		 echo 1 > /sys/class/misc/dw-xdata-pcie.<device>/write
+		or
+		 echo 0 > /sys/class/misc/dw-xdata-pcie.<device>/write
+
+		The user can read the current PCIe link throughput generated
+		through this generator in MB/s.
+
+		Usage e.g.
+		 cat /sys/class/misc/dw-xdata-pcie.<device>/write
+		 204
+
+		The file is read and write.
+
+What:		/sys/class/misc/dw-xdata-pcie.<device>/read
+Date:		April 2021
+KernelVersion:	5.13
+Contact:	Gustavo Pimentel <gustavo.pimentel@synopsys.com>
+Description:	Allows the user to enable the PCIe traffic generator which
+		will create read TLPs frames - from the Endpoint to the Root
+		Complex direction or to disable the PCIe traffic generator
+                in all directions.
+
+		Write y/1/on to enable, n/0/off to disable
+
+		Usage e.g.
+		 echo 1 > /sys/class/misc/dw-xdata-pcie.<device>/read
+		or
+		 echo 0 > /sys/class/misc/dw-xdata-pcie.<device>/read
+
+		The user can read the current PCIe link throughput generated
+		through this generator in MB/s.
+
+		Usage e.g.
+		 cat /sys/class/misc/dw-xdata-pcie.<device>/read
+		 199
+
+		The file is read and write.