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/irqchip/irq-gic-v3.c b/drivers/irqchip/irq-gic-v3.c
index 324f280..850842f 100644
--- a/drivers/irqchip/irq-gic-v3.c
+++ b/drivers/irqchip/irq-gic-v3.c
@@ -965,10 +965,10 @@ static void gic_cpu_sys_reg_init(void)
 		case 7:
 			write_gicreg(0, ICC_AP0R3_EL1);
 			write_gicreg(0, ICC_AP0R2_EL1);
-		/* Fall through */
+			fallthrough;
 		case 6:
 			write_gicreg(0, ICC_AP0R1_EL1);
-		/* Fall through */
+			fallthrough;
 		case 5:
 		case 4:
 			write_gicreg(0, ICC_AP0R0_EL1);
@@ -982,10 +982,10 @@ static void gic_cpu_sys_reg_init(void)
 	case 7:
 		write_gicreg(0, ICC_AP1R3_EL1);
 		write_gicreg(0, ICC_AP1R2_EL1);
-		/* Fall through */
+		fallthrough;
 	case 6:
 		write_gicreg(0, ICC_AP1R1_EL1);
-		/* Fall through */
+		fallthrough;
 	case 5:
 	case 4:
 		write_gicreg(0, ICC_AP1R0_EL1);