iio: cros_ec_accel_legacy: Add Read Only frequency entries

Report to user space that 10Hz is the sampling frequency of
the accelerometers in legacy mode, and it can not be changed.

Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
Reviewed-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>