ptp: ocp: don't allow on S390

Fix kconfig warning on arch/s390/:

WARNING: unmet direct dependencies detected for SERIAL_8250
  Depends on [n]: TTY [=y] && HAS_IOMEM [=y] && !S390 [=y]
  Selected by [m]:
  - PTP_1588_CLOCK_OCP [=m] && PTP_1588_CLOCK [=m] && HAS_IOMEM [=y] && PCI [=y] && SPI [=y] && I2C [=m] && MTD [=m]

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Richard Cochran <richardcochran@gmail.com>
Cc: Jonathan Lemon <jonathan.lemon@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>