dt-bindings: iio:imu:mpu6050: support more interrupt types

Document that the hardware supports falling edge, rising edge, level
low, and level high interrupt types, rather than just rising edge.

The language used is the same as that in st_lsm6dsx.txt.

Signed-off-by: Martin Kelly <mkelly@xevo.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Jean-Baptiste Maneyrol <jmaneyrol@invensense.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>