staging: fsl-dpaa2/rtc: fix PTP dependency

We can't select PTP_1588_CLOCK when posix timers are completely
disabled:

WARNING: unmet direct dependencies detected for PTP_1588_CLOCK
  Depends on [n]: NET [=y] && POSIX_TIMERS [=n]
  Selected by [y]:
  - FSL_DPAA2_PTP_CLOCK [=y] && STAGING [=y] && FSL_DPAA2_ETH [=y]

This adds the necessary dependency.

Fixes: 9bdf43b3 ("staging: fsl-dpaa2/rtc: add rtc driver")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>