net: lan966x: Fix when CONFIG_PTP_1588_CLOCK is compiled as module

When CONFIG_PTP_1588_CLOCK is compiled as a module, then the linking of
the lan966x fails because it can't find references to the following
functions 'ptp_clock_index', 'ptp_clock_register' and
'ptp_clock_unregister'

The fix consists in adding CONFIG_PTP_1588_CLOCK_OPTIONAL as a
dependency for the driver.

Fixes: d0964594 ("net: lan966x: Add support for ptp clocks")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>