iio: magn: Split bmc150 driver in common/i2c parts

This is useful for easily adding SPI support in later patches.

Now bmc150_magn exports core functions to be used by I2C/SPI drivers
instances. For the moment only I2C driver is supported.

Signed-off-by: Daniel Baluta <daniel.baluta@intel.com>
Acked-by: Irina Tirdea <irina.tirdea@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>