hwrng: exynos - Add SMC based TRNG operation

hwrng: exynos - Add SMC based TRNG operation

On some Exynos chips like Exynos850 the access to Security Sub System
(SSS) registers is protected with TrustZone, and therefore only possible
from EL3 monitor software. The Linux kernel is running in EL1, so the
only way for the driver to obtain TRNG data is via SMC calls to EL3
monitor. Implement such SMC operation and use it when EXYNOS_SMC flag is
set in the corresponding chip driver data.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>