cpsw/netcp: refine cpts dependency

Tony Lindgren reports a kernel oops that resulted from my compile-time
fix on the default config. This shows two problems:

a) configurations that did not already enable PTP_1588_CLOCK will
   now miss the cpts driver

b) when cpts support is disabled, the driver crashes. This is a
   preexisting problem that we did not notice before my patch.

While the second problem is still being investigated, this modifies
the dependencies again, getting us back to the original state, with
another 'select NET_PTP_CLASSIFY' added in to avoid the original
link error we got, and the 'depends on POSIX_TIMERS' to hide
the CPTS support when turning it on would be useless.

Cc: stable@vger.kernel.org # 4.11 needs this
Fixes: 07fef362 ("cpsw/netcp: cpts depends on posix_timers")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Tested-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: David S. Miller <davem@davemloft.net>