iio: imu: inv_icm42600: add SPI driver for inv_icm42600 driver

Add SPI driver for InvenSense ICM-426xxx devices.

Configure bus signal slew rates as indicated in the datasheet.

Signed-off-by: Jean-Baptiste Maneyrol <jmaneyrol@invensense.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>