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: H. Nikolaus Schaller <hns@goldelico.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>