Add DT support for the wl1271 SPI WiFi.

Add documentation file for the wl1271 SPI WiFi.
Signed-off-by: Uri Mashiach <uri.mashiach@compulab.co.il>
Acked-by: Rob Herring <robh@kernel.org>
Tested-By: Sebastian Reichel <sre@kernel.org>
Reviewed-by: Grygorii Strashko <grygorii.strashko@ti.com>
Acked-by: Igor Grinberg <grinberg@compulab.co.il>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>