Rename decoding signals to be more consistent

r/n now always refers to 8bit registers, rr/nn to 16bit.

rtl/cpu/cpu_pkg.svh

@@ -85,6 +85,7 @@ package cpu_pkg;
         ADR_SRC_HL,
         ADR_SRC_REG8, // extended with FF
         ADR_SRC_REG16,
+        ADR_SRC_OPERAND8, // extended with FF
         ADR_SRC_OPERAND16
     } adr_src_t;