fw: rivos: Add Rivos specific changes under FW_RIVOS

Rivos specific firmware need requires the following setup before
opensbi firmware execution starts.

1. R-code setup
2. MCRR/MDRR setup from LRAM
3. PMARR setup

Currently, fw_rivos does all 3 operations if FW_RIVOS=y is provided
at runtime. Eventually, #1 should be done by separate R-code stage
code as we need to perform more than jump to LRAM.

Ideally, #2 & #3 should be done stage prior to OpenSBI. In upstream
boot flow U-BOOT SPL can be used to support that. However, Rivos
firmware has already multiple stages boot flow defined. Eventually,
we should be able to drop the code under FW_RIVOS and just load
opensbi as last stage resident M-mode firmware (fw4-m).

Signed-off-by: Atish Patra <atishp@rivosinc.com>