sensor: ak8975: Convert to DTS

Convert ak8975 sensor driver to utilize device tree.

This also supports the ak8975 embedded in a invensense MPU9150.  In such
a case the device tree node should look something like, where the ak8975
is a child of the mpu9150.

        mpu9150@68 {
                compatible = "invensense,mpu9150";
                reg = <0x68>;
                label = "mpu9150";
                #address-cells = <1>;
                #size-cells = <0>;
                ak8975@c {
                        compatible = "asahi-kasei,ak8975";
                        reg = <0xc>;
                        label = "ak8975";
                };
        };

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>