iio: accel: mma9551: split driver to expose mma955x api

Freescale has the MMA955xL family of devices that use the
same communication protocol (based on i2c messages):
http://www.freescale.com/files/sensors/doc/data_sheet/MMA955xL.pdf

.

To support more devices from this family, we need to split the
mma9551 driver so we can export the common functions that will
be used by other mma955x drivers.

Signed-off-by: Irina Tirdea <irina.tirdea@intel.com>
Reviewed-by: Vlad Dogaru <vlad.dogaru@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>