spi/orion: add device tree binding

Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Jason Cooper <jason@lakedaemon.net>
Acked-by: Sebastian Hesselbarth <sebastian.hesselbarth@googlemail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>