disas/sh4: Add missing fallthrough annotations

Add fallthrough annotations to be able to compile the code without
warnings with -Wimplicit-fallthrough. Looking at the code, it seems
like the fallthrough is indeed intended here, so the comments should
be appropriate.

Message-Id: <20200630055953.9309-1-thuth@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>

disas/sh4.c

@@ -1963,6 +1963,7 @@ print_insn_sh (bfd_vma memaddr, struct disassemble_info *info)
 		  fprintf_fn (stream, "xd%d", rn & ~1);
 		  break;
 		}
+	      /* fallthrough */
 	    case D_REG_N:
 	      fprintf_fn (stream, "dr%d", rn);
 	      break;
@@ -1972,6 +1973,7 @@ print_insn_sh (bfd_vma memaddr, struct disassemble_info *info)
 		  fprintf_fn (stream, "xd%d", rm & ~1);
 		  break;
 		}
+	      /* fallthrough */
 	    case D_REG_M:
 	      fprintf_fn (stream, "dr%d", rm);
 	      break;