Documentation: misc-devices: Add Documentation for dw-xdata-pcie driver

Add Documentation for dw-xdata-pcie driver.
Signed-off-by: Gustavo Pimentel <gustavo.pimentel@synopsys.com>
Link: https://lore.kernel.org/r/764b9bf744d7fe20c7a216019eef8ddf482c1bd7.1617016509.git.gustavo.pimentel@synopsys.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/misc-devices/dw-xdata-pcie.rst

+.. SPDX-License-Identifier: GPL-2.0
+
+===========================================================================
+Driver for Synopsys DesignWare PCIe traffic generator (also known as xData)
+===========================================================================
+
+This driver should be used as a host-side (Root Complex) driver and Synopsys
+DesignWare prototype that includes this IP.
+
+The "dw-xdata-pcie" driver can be used to enable/disable PCIe traffic
+generator in either direction (mutual exclusion) besides allowing the
+PCIe link performance analysis.
+
+The interaction with this driver is done through the module parameter and
+can be changed in runtime. The driver outputs the requested command state
+information to /var/log/kern.log or dmesg.
+
+Request write TLPs traffic generation - Root Complex to Endpoint direction
+- Command:
+	echo 1 > /sys/class/misc/dw-xdata-pcie/write
+
+Get write TLPs traffic link throughput in MB/s
+- Command:
+        cat /sys/class/misc/dw-xdata-pcie/write
+- Output example:
+	204
+
+Request read TLPs traffic generation - Endpoint to Root Complex direction:
+- Command:
+	echo 1 > /sys/class/misc/dw-xdata-pcie/read
+
+Get read TLPs traffic link throughput in MB/s
+- Command:
+        cat /sys/class/misc/dw-xdata-pcie/read
+- Output example:
+	199
+
+Request to stop any current TLP transfer:
+- Command:
+	echo 1 > /sys/class/misc/dw-xdata-pcie/stop