iio: accel: add support for mxc4005 accelerometer

This patch adds support for Memsic MXC4005XC 3-axis accelerometer. The
current implementation is a minimal one as it adds raw readings for the
three axes and setting scale from userspace.

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