cpts: Fix build error caused by include of plat/clock.h

Commit 87c0e764 (cpts: introduce time stamping code and a PTP hardware clock)
mistakenly included plat/clock.h that should not be included by drivers
even if it exists.

Otherwise we get the following error with at least omap2plus_defconfig:

drivers/net/ethernet/ti/cpts.c:30:24: error: plat/clock.h: No such file or directory

Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: David S. Miller <davem@davemloft.net>