net: micrel: fix KS8851_MLL Kconfig

KS8851_MLL selects MICREL_PHY, which depends on PTP_1588_CLOCK_OPTIONAL,
so make KS8851_MLL also depend on PTP_1588_CLOCK_OPTIONAL since
'select' does not follow any dependency chains.

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_MLL [=y] && NETDEVICES [=y] && ETHERNET [=y] && NET_VENDOR_MICREL [=y] && HAS_IOMEM [=y]

ld: drivers/net/phy/micrel.o: in function `lan8814_ts_info':
micrel.c:(.text+0xb35): undefined reference to `ptp_clock_index'
ld: drivers/net/phy/micrel.o: in function `lan8814_probe':
micrel.c:(.text+0x2586): undefined reference to `ptp_clock_register'

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>