kernel/ff: Refactor FfData to enable FFs with async load.

- *_en is split into *_ce (clock enable) and *_aload (async load aka
  latch gate enable), so both can be present at once
- has_d is removed
- has_gclk is added (to have a clear marker for $ff)
- d_is_const and val_d leftovers are removed
- async2sync, clk2fflogic, opt_dff are updated to operate correctly on
  FFs with async load

passes/techmap/dffunmap.cc

@@ -84,7 +84,7 @@ struct DffunmapPass : public Pass {
 					continue;
 
 				if (ce_only) {
-					if (!ff.has_en)
+					if (!ff.has_ce)
 						continue;
 					ff.unmap_ce(mod);
 				} else if (srst_only) {
@@ -92,7 +92,7 @@ struct DffunmapPass : public Pass {
 						continue;
 					ff.unmap_srst(mod);
 				} else {
-					if (!ff.has_en && !ff.has_srst)
+					if (!ff.has_ce && !ff.has_srst)
 						continue;
 					ff.unmap_ce_srst(mod);
 				}