i.MX: add an emulation for RNGC

Add an emulation for the RNGC random number generator and the compatible
RNGB variant. These peripherals are included (at least) in imx25 and
imx35 chipsets.

The emulation supports the initial self test, reseeding the prng and
reading random numbers.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>