tests: ptp: Fixed PTP clock test failure on frdm_k64f

ptp_clock_test was failing when trying to access the ptp clock from user
mode. This was due to an arbitrary initialization priority between the
virtual PTP clock devices created by the ptp clocking test and the
physical PTP clock on the frdm_k64f board. Since the ptp test
initializes the virtual clock with the same device name as the physical
clock, if the physical clock was initialized first this test would
incorrectly reference the physical device during testing, instead of the
virtual one.

This fix simply defines the virtual ptp clock with a custom name, so
that the test will always locate the correct ptp clock.
Fixes #38731

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>