dt-bindings: iio: magnetometer: add MEMSIC MMC5983MA

dt-bindings: iio: magnetometer: add MEMSIC MMC5983MA

Add a Devicetree binding for the MEMSIC MMC5983MA 3-axis
magnetometer.

MMC5983MA is not register-compatible with the existing MEMSIC
magnetometer drivers. It has a different register map, 18-bit output
data format, and I2C/SPI transport support.

Reviewed-by: David Lechner <dlechner@baylibre.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Vladislav Kulikov <vlad.kulikov.c@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>