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>

drivers/mfd/syscon.c

@@ -95,7 +95,7 @@ static struct syscon *of_syscon_register(struct device_node *np, bool check_clk)
 			break;
 		default:
 			pr_err("Failed to retrieve valid hwlock: %d\n", ret);
-			/* fall-through */
+			fallthrough;
 		case -EPROBE_DEFER:
 			goto err_regmap;
 		}