net: micrel: Fix KS8851 Kconfig

KS8851 selects MICREL_PHY, which depends on PTP_1588_CLOCK_OPTIONAL, so
make KS8851 also depend on PTP_1588_CLOCK_OPTIONAL.

Fixes kconfig warning and build errors:

WARNING: unmet direct dependencies detected for MICREL_PHY
  Depends on [m]: NETDEVICES [=y] && PHYLIB [=y] && PTP_1588_CLOCK_OPTIONAL [=m]
    Selected by [y]:
      - KS8851 [=y] && NETDEVICES [=y] && ETHERNET [=y] && NET_VENDOR_MICREL [=y] && SPI [=y]

ld.lld: error: undefined symbol: ptp_clock_register referenced by micrel.c
net/phy/micrel.o:(lan8814_probe) in archive drivers/built-in.a
ld.lld: error: undefined symbol: ptp_clock_index referenced by micrel.c
net/phy/micrel.o:(lan8814_ts_info) in archive drivers/built-in.a

Reported-by: kernel test robot <lkp@intel.com>
Fixes: ece19502

 ("net: phy: micrel: 1588 support for LAN8814 phy")
Signed-off-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/20220405065936.4105272-1-horatiu.vultur@microchip.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>