iio: cros_ec: Switch to SPDX identifier.

Adopt the SPDX license identifier headers to ease license compliance
management. Also fix MODULE_LICENSE for cros_ec_accel_legacy to match
the SPDX and boiler plate license.

Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>