dt: bindings: add MARVELL's sd8xxx wireless device

Add device tree binding documentation for MARVELL's sd8xxx
(sd8897 and sd8997) wlan chip.

Signed-off-by: Xinming Hu <huxm@marvell.com>
Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>