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.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>