[entropy_src/rtl] bug fixes for rng interface FIFO
[entropy_src/rtl] bug fixes for rng interface FIFO

Changes made to the AST RNG interface controls so that back-to-back valids will be paused one cycle.
Also, the number of valids needed for a certain seed size match.
For example, a boot seed of 384 bits needs precisely 384 input valids signaled.

Signed-off-by: Mark Branstad <d7ca710cc6ec5dc810c866dae5369b19445fbeef@wdc.com>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan