[csrng/rtl] prevent x's from entering prim_arbiter_ppc_acmd
[csrng/rtl] prevent x's from entering prim_arbiter_ppc_acmd

Changing the data bus mux that feeds the app cmd arbiter to send zeros instead of x's when the mop signal is active.

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

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