net: ti: icssg-prueth: add PTP_1588_CLOCK_OPTIONAL dependency

net: ti: icssg-prueth: add PTP_1588_CLOCK_OPTIONAL dependency

The new icssg-prueth driver needs the same dependency as the other parts
that use the ptp-1588:

WARNING: unmet direct dependencies detected for TI_ICSS_IEP
  Depends on [m]: NETDEVICES [=y] && ETHERNET [=y] && NET_VENDOR_TI [=y] && PTP_1588_CLOCK_OPTIONAL [=m] && TI_PRUSS [=y]
  Selected by [y]:
  - TI_PRUETH [=y] && NETDEVICES [=y] && ETHERNET [=y] && NET_VENDOR_TI [=y] && PRU_REMOTEPROC [=y] && NET_SWITCHDEV [=y]

Add the correct dependency on the two drivers missing it, and remove
the pointless 'imply' in the process.

Fixes: e654b85a693e ("net: ti: icssg-prueth: Add ICSSG Ethernet driver for AM65x SR1.0 platforms")
Fixes: 511f6c1ae093 ("net: ti: icssm-prueth: Adds ICSSM Ethernet driver")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Vadim Fedorenko <vadim.fedorenko@linux.dev>
Link: https://patch.msgid.link/20251204100138.1034175-1-arnd@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>