iio: accel: add Freescale MMA7455L/MMA7456L 3-axis accelerometer driver

Add support for Freescale MMA7455L/MMA7456L 3-axis in 10-bit mode for
I2C and SPI bus. This rather simple driver that currently doesn't
support all the hardware features of MMA7455L/MMA7456L.

Tested on Embedded Artist's LPC4357 Dev Kit with MMA7455L on I2C bus.

Data sheets for the two devices can be found here:
http://cache.freescale.com/files/sensors/doc/data_sheet/MMA7455L.pdf
http://cache.freescale.com/files/sensors/doc/data_sheet/MMA7456L.pdf



Signed-off-by: Joachim Eastwood <manabian@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>