iio: bmg160: Separate i2c and core driver

This patch separates the core driver using regmap and the i2c driver
which creates the i2c regmap. Also in the Kconfig file BMG160 and
BMG160_I2C are separate now.

Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Reviewed-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>