[TRNG] add optional/configurable data FIFO

[stnolting]

This PR adds an optional and configurable data FIFO to the processor's true random number generator module (TRNG).

The TRNG requires at least 16x8=128 cycles to generate a new RND data bytes (see stnolting/neoTRNG for more information). The new optional FIFO provides something like a "random data pool" by constantly sampling data from the TRNG core.

• The TRNG FIFO is configured by the new top generic IO_TRNG_FIFO: default value is 1, minimal value is 1, has to be a power of two.
• The FIFO can be cleared at any time either by disabling and re-enabling the TRNG module or via the new control register bit TRNG_CTRL_FIFO_CLR (NEORV32_TRNG.CTRL bit 28; write-only, auto clears).