net: sparx5: depends on PTP_1588_CLOCK_OPTIONAL

Fix build errors when PTP_1588_CLOCK=m and SPARX5_SWTICH=y.

arc-linux-ld: drivers/net/ethernet/microchip/sparx5/sparx5_ethtool.o: in function `sparx5_get_ts_info':
sparx5_ethtool.c:(.text+0x146): undefined reference to `ptp_clock_index'
arc-linux-ld: sparx5_ethtool.c:(.text+0x146): undefined reference to `ptp_clock_index'
arc-linux-ld: drivers/net/ethernet/microchip/sparx5/sparx5_ptp.o: in function `sparx5_ptp_init':
sparx5_ptp.c:(.text+0xd56): undefined reference to `ptp_clock_register'
arc-linux-ld: sparx5_ptp.c:(.text+0xd56): undefined reference to `ptp_clock_register'
arc-linux-ld: drivers/net/ethernet/microchip/sparx5/sparx5_ptp.o: in function `sparx5_ptp_deinit':
sparx5_ptp.c:(.text+0xf30): undefined reference to `ptp_clock_unregister'
arc-linux-ld: sparx5_ptp.c:(.text+0xf30): undefined reference to `ptp_clock_unregister'
arc-linux-ld: sparx5_ptp.c:(.text+0xf38): undefined reference to `ptp_clock_unregister'
arc-linux-ld: sparx5_ptp.c:(.text+0xf46): undefined reference to `ptp_clock_unregister'
arc-linux-ld: drivers/net/ethernet/microchip/sparx5/sparx5_ptp.o:sparx5_ptp.c:(.text+0xf46): more undefined references to `ptp_clock_unregister' follow

Fixes: 3cfa11ba

 ("net: sparx5: add the basic sparx5 driver")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: kernel test robot <lkp@intel.com>
Cc: Horatiu Vultur <horatiu.vultur@microchip.com>
Cc: UNGLinuxDriver@microchip.com
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: Steen Hegelund <steen.hegelund@microchip.com>
Cc: Bjarni Jonasson <bjarni.jonasson@microchip.com>
Cc: Lars Povlsen <lars.povlsen@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>