net: macb: Remove redundant ptp clock selection

net: macb: Remove redundant ptp clock selection

PTP_1588_CLOCK is already implied when MACB_USE_HWSTAMP is selected.
Remove the unconditional selection under macb config.
Also using the imply instead of select avoid errors on configurations
such as:

	make ARCH=i386

All errors (new ones prefixed by >>):

   drivers/ptp/ptp_clock.o: In function `ptp_clock_register':
>> drivers/ptp/ptp_clock.c:274: undefined reference to `posix_clock_register'
   drivers/ptp/ptp_clock.o: In function `ptp_clock_unregister':
>> drivers/ptp/ptp_clock.c:320: undefined reference to `posix_clock_unregister'

Signed-off-by: Harini Katakam <harini.katakam@xilinx.com>
Acked-by: Radhey Shyam Pandey <radheys@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>