hw/misc: Add Exynos4210 Pseudo Random Number Generator

hw/misc: Add Exynos4210 Pseudo Random Number Generator

Add emulation for Exynos4210 Pseudo Random Number Generator which could
work on fixed seeds or with seeds provided by True Random Number
Generator block inside the SoC.

Implement only the fixed seeds part of it in polling mode (no
interrupts).

Emulation tested with two independent Linux kernel exynos-rng drivers:
1. New kcapi-rng interface (targeting Linux v4.12),
2. Old hwrng inteface
   # echo "exynos" > /sys/class/misc/hw_random/rng_current
   # dd if=/dev/hwrng of=/dev/null bs=1 count=16

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Message-id: 20170425180609.11004-1-krzk@kernel.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
[PMM: wrapped a few overlong lines; more efficient implementation
 of exynos4210_rng_seed_ready()]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>