powerpc: 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 <[e-mail]>
Signed-off-by: Michael Ellerman <[e-mail]>
Link: https://lore.kernel.org/r/20200727224201.GA10133@embeddedor

arch/powerpc/kernel/align.c

···
 			ret |= __get_user_inatomic(temp.v[1], p++);
 			ret |= __get_user_inatomic(temp.v[2], p++);
 			ret |= __get_user_inatomic(temp.v[3], p++);
-			/* fall through */
+			fallthrough;
 		case 4:
 			ret |= __get_user_inatomic(temp.v[4], p++);
 			ret |= __get_user_inatomic(temp.v[5], p++);
-			/* fall through */
+			fallthrough;
 		case 2:
 			ret |= __get_user_inatomic(temp.v[6], p++);
 			ret |= __get_user_inatomic(temp.v[7], p++);
···
 			ret |= __put_user_inatomic(data.v[1], p++);
 			ret |= __put_user_inatomic(data.v[2], p++);
 			ret |= __put_user_inatomic(data.v[3], p++);
-			/* fall through */
+			fallthrough;
 		case 4:
 			ret |= __put_user_inatomic(data.v[4], p++);
 			ret |= __put_user_inatomic(data.v[5], p++);
-			/* fall through */
+			fallthrough;
 		case 2:
 			ret |= __put_user_inatomic(data.v[6], p++);
 			ret |= __put_user_inatomic(data.v[7], p++);

arch/powerpc/platforms/powermac/feature.c

···
 	case 2:
 		if (macio->type == macio_shasta)
 			break;
-		/* fall through */
+		fallthrough;
 	default:
 		return -ENODEV;
 	}

arch/powerpc/platforms/powernv/opal-async.c

···
 	 */
 	case ASYNC_TOKEN_DISPATCHED:
 		opal_async_tokens[token].state = ASYNC_TOKEN_ABANDONED;
-		/* Fall through */
+		fallthrough;
 	default:
 		rc = 1;
 	}

arch/powerpc/platforms/pseries/hvcserver.c

···
 		case H_LONG_BUSY_ORDER_10_SEC:
 		case H_LONG_BUSY_ORDER_100_SEC:
 			return -EBUSY;
-		case H_FUNCTION: /* fall through */
+		case H_FUNCTION:
 		default:
 			return -EPERM;
 	}

arch/powerpc/xmon/xmon.c

···
 		subcmd = inchar();
 		if (isxdigit(subcmd) || subcmd == '\n')
 			termch = subcmd;
-		/* fall through */
+		fallthrough;
 	case 'f':
 		scanhex(&num);
 		if (num >= XMON_NUM_SPUS || !spu_info[num].spu) {