hw/misc: Add npcm7xx random number generator

hw/misc: Add npcm7xx random number generator

The RNG module returns a byte of randomness when the Data Valid bit is
set.

This implementation ignores the prescaler setting, and loads a new value
into RNGD every time RNGCS is read while the RNG is enabled and random
data is available.

A qtest featuring some simple randomness tests is included.

Reviewed-by: Tyrone Ting <kfting@nuvoton.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Havard Skinnemoen <hskinnemoen@google.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>