ARM: dts: stm32: use pll4_p for ethernet PTP clock - #4

IEEE 1588 support needs a clock defined as ptp_ref and this uses pll4_p as the parent clock

before:
$ ./testptp -g ; sleep 1 ; ./testptp -g
clock time: 0.000000000 or Wed Dec 31 18:00:00 1969
clock time: 0.000000000 or Wed Dec 31 18:00:00 1969

after:
$ ./testptp -g ; sleep 1 ; ./testptp -g
clock time: 1587623094.279486315 or Thu Apr 23 01:24:54 2020
clock time: 1587623095.292978915 or Thu Apr 23 01:24:55 2020