net: ti: icssg-prueth: add PTP dependency

The driver can now use PTP if enabled but fails to link built-in
if PTP is a loadable module:

aarch64-linux-ld: drivers/net/ethernet/ti/icssg/icss_iep.o: in function `icss_iep_get_ptp_clock_idx':
icss_iep.c:(.text+0x200): undefined reference to `ptp_clock_index'

Add the usual dependency to avoid this.

Fixes: 186734c1

 ("net: ti: icssg-prueth: add packet timestamping and ptp support")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: MD Danish Anwar <danishanwar@ti.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>