net/mlx5e: clock.c depends on CONFIG_PTP_1588_CLOCK

lib/clock.c includes clock related functions which require ptp support.
Thus compile out lib/clock.c and add the needed function stubs in case
kconfig CONFIG_PTP_1588_CLOCK is off.

Signed-off-by: Moshe Shemesh <moshe@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>