media: Add document file for CXD2880 SPI I/F

This is the document file for Sony CXD2880 DVB-T2/T tuner + demodulator.
It contains the description of the SPI adapter binding.
Signed-off-by: Yasunari Takiguchi <Yasunari.Takiguchi@sony.com>
Signed-off-by: Masayuki Yamamoto <Masayuki.Yamamoto@sony.com>
Signed-off-by: Hideki Nozawa <Hideki.Nozawa@sony.com>
Signed-off-by: Kota Yonezawa <Kota.Yonezawa@sony.com>
Signed-off-by: Toshihiko Matsumoto <Toshihiko.Matsumoto@sony.com>
Signed-off-by: Satoshi Watanabe <Satoshi.C.Watanabe@sony.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/devicetree/bindings/media/spi/sony-cxd2880.txt

+Sony CXD2880 DVB-T2/T tuner + demodulator driver SPI adapter
+
+Required properties:
+- compatible: Should be "sony,cxd2880".
+- reg: SPI chip select number for the device.
+- spi-max-frequency: Maximum bus speed, should be set to <55000000> (55MHz).
+
+Example:
+
+cxd2880@0 {
+	compatible = "sony,cxd2880";
+	reg = <0>; /* CE0 */
+	spi-max-frequency = <55000000>; /* 55MHz */
+};