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: NDaniel Baluta <daniel.baluta@intel.com>
Acked-by: NIrina Tirdea <irina.tirdea@intel.com>
Signed-off-by: NJonathan Cameron <jic23@kernel.org>