[ot] hw/opentitan: ot_rom_ctrl: Remove S&P from data scrambling

In Earlgrey, the S&P layer has been removed from the data scrambling
logic, which affects the scrambling of the generated ROM. The
functionality is removed entirely from the ROM Control to improve error
detection guarantees, interactions with ECC, and timing.

This commit removes the now-redundant logic related to the S&P layer,
instead directly unscrambling words by XORing the keystream and the
`scr`/`in` data.

hw/opentitan/ot_rom_ctrl.c

@@ -121,10 +121,6 @@ static const char *REG_NAMES[REGS_COUNT] = {
 static const uint8_t SBOX4[16u] = {
     12u, 5u, 6u, 11u, 9u, 0u, 10u, 13u, 3u, 14u, 15u, 8u, 4u, 7u, 1u, 2u
 };
-
-static const uint8_t SBOX4_INV[16u] = {
-    5u, 14u, 15u, 8u, 12u, 1u, 2u, 13u, 11u, 4u, 6u, 3u, 0u, 7u, 9u, 10u
-};
 /* clang-format on */
 
 static const OtKMACAppCfg KMAC_APP_CFG =
@@ -281,36 +277,12 @@ static uint64_t ot_rom_ctrl_subst_perm_enc(uint64_t in, uint64_t key,
     return state;
 }
 
-static uint64_t ot_rom_ctrl_subst_perm_dec(uint64_t in, uint64_t key,
-                                           unsigned width, unsigned num_rounds)
-{
-    uint64_t state = in;
-
-    for (unsigned ix = 0; ix < num_rounds; ix++) {
-        state ^= key;
-        state = ot_rom_ctrl_perm(state, width, true);
-        state = ot_rom_ctrl_flip(state, width);
-        state = ot_rom_ctrl_sbox(state, width, SBOX4_INV);
-    }
-
-    state ^= key;
-
-    return state;
-}
-
 static unsigned ot_rom_ctrl_addr_sp_enc(const OtRomCtrlState *s, unsigned addr)
 {
     return ot_rom_ctrl_subst_perm_enc(addr, s->addr_nonce, s->addr_width,
                                       OT_ROM_CTRL_NUM_ADDR_SUBST_PERM_ROUNDS);
 }
 
-static uint64_t ot_rom_ctrl_data_sp_dec(const OtRomCtrlState *s, uint64_t in)
-{
-    (void)s;
-    return ot_rom_ctrl_subst_perm_dec(in, 0, OT_ROM_CTRL_WORD_BITS,
-                                      OT_ROM_CTRL_NUM_DATA_SUBST_PERM_ROUNDS);
-}
-
 static uint64_t
 ot_rom_ctrl_get_keystream(const OtRomCtrlState *s, unsigned addr)
 {
@@ -438,8 +410,7 @@ static uint64_t
 ot_rom_ctrl_unscramble_word(const OtRomCtrlState *s, unsigned addr, uint64_t in)
 {
     uint64_t keystream = ot_rom_ctrl_get_keystream(s, addr);
-    uint64_t sp = ot_rom_ctrl_data_sp_dec(s, in);
-    return keystream ^ sp;
+    return keystream ^ in;
 }
 
 static uint32_t ot_rom_ctrl_verify_ecc_39_32_u32(