crypto: exynos - Introduce mutex to prevent concurrent access to hardware

Hardware operations like reading random numbers and setting a seed need
to be conducted in a single thread. Therefore a mutex is required to
prevent multiple threads (processes) from accessing the hardware at the
same time.

The sequence of mutex_lock() and mutex_unlock() in the exynos_rng_reseed()
function enables switching between different threads waiting for the
driver to generate random numbers for them.

Signed-off-by: Łukasz Stelmach <l.stelmach@samsung.com>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>