cpsw/netcp: cpts depends on posix_timers

With posix timers having become optional, we get a build error with
the cpts time sync option of the CPSW driver:

drivers/net/ethernet/ti/cpts.c: In function 'cpts_find_ts':
drivers/net/ethernet/ti/cpts.c:291:23: error: implicit declaration of function 'ptp_classify_raw';did you mean 'ptp_classifier_init'? [-Werror=implicit-function-declaration]

This adds a hard dependency on PTP_CLOCK to avoid the problem, as
building it without PTP support makes no sense anyway.

Fixes: baa73d9e

 ("posix-timers: Make them configurable")
Cc: Nicolas Pitre <nicolas.pitre@linaro.org>
Cc: stable@vger.kernel.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>