treewide: Use fallthrough pseudo-keyword

Replace the existing /* fall through */ comments and its variants with
the new pseudo-keyword macro fallthrough[1]. Also, remove unnecessary
fall-through markings when it is the case.

[1] https://www.kernel.org/doc/html/v5.7/process/deprecated.html?highlight=fallthrough#implicit-switch-case-fall-through

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
diff --git a/drivers/bus/ti-sysc.c b/drivers/bus/ti-sysc.c
index fb5a901..efb088d 100644
--- a/drivers/bus/ti-sysc.c
+++ b/drivers/bus/ti-sysc.c
@@ -1849,7 +1849,7 @@ static int sysc_clockdomain_init(struct sysc *ddata)
 	switch (ddata->nr_clocks) {
 	case 2:
 		ick = ddata->clocks[SYSC_ICK];
-		/* fallthrough */
+		fallthrough;
 	case 1:
 		fck = ddata->clocks[SYSC_FCK];
 		break;