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