[RISCV] Rename sf_vcix_state to sf.vcix_state. NFC (llvm#107115)

This PR: llvm#106995 names the
vendor CSR in a wrong way, it should be `sf.` rather than `sf_` for
prefix.

Author: 4vtomat

llvm/lib/Target/RISCV/RISCVRegisterInfo.td

@@ -665,4 +665,4 @@ def FRM    : RISCVReg<0, "frm">;
 def SSP    : RISCVReg<0, "ssp">;
 
 // Dummy SiFive VCIX state register
-def SF_VCIX_STATE : RISCVReg<0, "sf_vcix_state">;
+def SF_VCIX_STATE : RISCVReg<0, "sf.vcix_state">;