Replace GPL license statement with SPDX GPL-2.0 license identifier and
correct the module license to GPLv2.

The license itself was a generic GPL because of copy-and-paste from old
drivers/char/hw_random/exynos-rng.c driver (on which this was based on).
However the module license indicated GPL-2.0 or later.  GPL-2.0 was
intended by author so fix up this mess.
Signed-off-by: NKrzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>