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.com


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