powerpc/52xx: Add fallthrough in mpc52xx_wdt_ioctl()

With gcc 10.3, there is this compiler error:

  compiler.h:56:26: error: this statement may fall through
  mpc52xx_gpt.c:586:2: note: here
    586 |  case WDIOC_GETTIMEOUT:
        |  ^~~~

So add the fallthrough pseudo keyword.
Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210601190200.2637776-1-trix@redhat.com

arch/powerpc/platforms/52xx/mpc52xx_gpt.c

@@ -582,6 +582,7 @@ static long mpc52xx_wdt_ioctl(struct file *file, unsigned int cmd,
 		if (ret)
 			break;
 		/* fall through and return the timeout */
+		fallthrough;
 
 	case WDIOC_GETTIMEOUT:
 		/* we need to round here as to avoid e.g. the following