[rram_ctrl] add seed_valid for keymgr

@gautschimi is already working on this.

Since Aug 14, 2026.

https://github.com/lowRISC/opentitan/blob/master/hw/ip/rram_ctrl/rtl/rram_ctrl_lcmgr.sv#L295-L318

The seeds are validated by a second read which can set a bit back to 0 because of an AND with the previous read.

The RRAM is already verifying every read operation. This check is maybe a bit obsolete.

The keymgr also requires a valid signal. This can set to one if the two values match.

```sv
assign valid[seed_idx][rd_idx] = seed_q[seed_idx][rd_idx] == rdata_i[BusWidth-1:0];

assign seeds_valid_o = &valid;
```