crypto: exynos - Improve performance of PRNG

Use memcpy_fromio() instead of custom exynos_rng_copy_random() function
to retrieve generated numbers from the registers of PRNG.

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>