dt: bindings: add TI's wilink wireless device

Add device tree binding documentation for TI's wilink
(wl12xx and wl18xx) wlan chip.

Signed-off-by: Eliad Peller <eliad@wizery.com>
Tested-by: Nikita Kiryanov <nikita@compulab.co.il>
Acked-by: Kalle Valo <kvalo@codeaurora.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>