Documentation: dt: wireless: update wl1251 for sdio

The standard method for sdio devices connected to
an sdio interface is to define them as a child node
like we can see with wlcore.

Signed-off-by: H. Nikolaus Schaller <hns@goldelico.com>
Acked-by: Kalle Valo <kvalo@codeaurora.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: <stable@vger.kernel.org> # v4.7+
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>