platform/chrome: cromeos_pstore: switch to SPDX identifier

Adopt the SPDX license identifier headers to ease license compliance
management. Also fix the module license mismatch and change the term
'Chrome OS' for 'ChromeOS' to be coherent with other drivers.

Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Reviewed-by: Guenter Roeck <groeck@chromium.org>