net: broadcom: Add PTP_1588_CLOCK_OPTIONAL dependency for BCMGENET under ARCH_BCM2835

commit 8d820bc9 ("net: broadcom: Fix BCMGENET Kconfig") fixes the build
that contain 99addbe3 ("net: broadcom: Select BROADCOM_PHY for BCMGENET")
and enable BCMGENET=y but PTP_1588_CLOCK_OPTIONAL=m, which otherwise
leads to a link failure. However this may trigger a runtime failure.

Fix the original issue by propagating the PTP_1588_CLOCK_OPTIONAL dependency
of BROADCOM_PHY down to BCMGENET.

Fixes: 8d820bc9 ("net: broadcom: Fix BCMGENET Kconfig")
Fixes: 99addbe3

 ("net: broadcom: Select BROADCOM_PHY for BCMGENET")
Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Suggested-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20221125115003.30308-1-yuehaibing@huawei.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>