iio: accel: add support for Memsic MXC6255XC sensor

This patch adds a minimal implementation for the Memsic MXC6255XC
orientation sensing accelerometer. The supported operations are reading
raw acceleration values for X/Y axis that can be scaled using the
exposed scale.

Signed-off-by: Teodora Baluta <teodora.baluta@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>