The mpu9250 sensor is 9-axis gyroscope+accelerometer+magnetometer combo
device which is made by InvenSense Inc. It can be accessed
via SPI bus with max frequency up to 20MHz. The recommended
SPI mode is the mode 3(CPOL=1, CPHA=1).

Required properties:
 - compatible        : Should be "invn,mpu9250".
 - reg               : the Chip select ID.
 - spi-max-frequency : the SPI bus max frequency.
 - invn,mu9250-irq   : the irq gpio.

Optional properties:
 - spi-cpol       : SPI MODE(CPOL).
 - spi-cpha       : SPI MODE(CPHA).

spi-cpol,spi-cpha,spi-max-frequency information is described in
Documentation/devicetree/bindings/spi/spi_qsd.txt.

Example:
	spi@7575000 {
		mpu9250-spi@0 {
			compatible = "invn,mpu9250";
			reg = <0>;
			spi-max-frequency = <8000000>;
			invn,mpu9250-irq = <&tlmm 79 0>;
			spi-cpol;
			spi-cpha;
		};
	};