ARM: dts: stm32: Add PTP clock to Ethernet controller

Add the PTP clock to the Ethernet controller. Otherwise, the driver uses the
main clock to derive the PTP frequency which is not necessarily the correct one.

Tested with linuxptp on Olimex STMP1-OLinuXino-LIME2.

Signed-off-by: Kurt Kanzenbach <kurt@linutronix.de>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>