net: Don't default Cavium PTP driver to 'y'

8c56df372b ("net: add support for Cavium PTP coprocessor") added the
Cavium PTP coprocessor driver and enabled it by default.  Remove the
"default y" because the driver only applies to Cavium ThunderX processors.

Fixes: 8c56df372b ("net: add support for Cavium PTP coprocessor")
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/cavium/Kconfig

@@ -54,7 +54,6 @@ config CAVIUM_PTP
 	tristate "Cavium PTP coprocessor as PTP clock"
 	depends on 64BIT && PCI
 	imply PTP_1588_CLOCK
-	default y
 	---help---
 	  This driver adds support for the Precision Time Protocol Clocks and
 	  Timestamping coprocessor (PTP) found on Cavium processors.