DTS: bindings: wl1251: mark ti,power-gpio as optional

It is now only useful for SPI interface.
Power control of SDIO mode is done through mmc core.

Suggested by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: NH. Nikolaus Schaller <hns@goldelico.com>
Signed-off-by: NKalle Valo <kvalo@codeaurora.org>

Documentation/devicetree/bindings/net/wireless/ti,wl1251.txt

@@ -9,11 +9,12 @@ Required properties:
 - spi-max-frequency : Maximum SPI clocking speed of device in Hz
 - interrupts :        Should contain interrupt line
 - vio-supply :        phandle to regulator providing VIO
-- ti,power-gpio :     GPIO connected to chip's PMEN pin
 
 Optional properties:
 - ti,wl1251-has-eeprom : boolean, the wl1251 has an eeprom connected, which
                          provides configuration data (calibration, MAC, ...)
+- ti,power-gpio :	 GPIO connected to chip's PMEN pin if operated in
+			 SPI mode
 - Please consult Documentation/devicetree/bindings/spi/spi-bus.txt
   for optional SPI connection related properties,