If CONFIG_PTP_1588_CLOCK_OPTIONAL=m and CONFIG_RENESAS_ETHER_SWITCH=y,
the following build error happened:

    aarch64-linux-ld: DWARF error: could not find abbrev number 60
    drivers/net/ethernet/renesas/rswitch.o: in function `rswitch_get_ts_info':
    rswitch.c:(.text+0x408): undefined reference to `ptp_clock_index'
    aarch64-linux-ld: DWARF error: could not find abbrev number 1190123
    drivers/net/ethernet/renesas/rcar_gen4_ptp.o: in function `rcar_gen4_ptp_register':
    rcar_gen4_ptp.c:(.text+0x4dc): undefined reference to `ptp_clock_register'
    aarch64-linux-ld: drivers/net/ethernet/renesas/rcar_gen4_ptp.o: in function `rcar_gen4_ptp_unregister':
    rcar_gen4_ptp.c:(.text+0x584): undefined reference to `ptp_clock_unregister'

To fix the issue, add "depends on PTP_1588_CLOCK_OPTIONAL" into the
Kconfig.
Reported-by: kernel test robot <lkp@intel.com>
Suggested-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 6c6fa1a0 ("net: ethernet: renesas: rswitch: Add R-Car Gen4 gPTP support")
Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Link: https://lore.kernel.org/r/20221110012720.3552060-1-yoshihiro.shimoda.uh@renesas.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>