ARM: dts: stm32: Add missing ETHCK clock to ethernet node

Add missing 'eth-ck' clock to the ethernet node. These clock are used to
generate external clock signal for the PHY in case 'st,eth_ref_clk_sel'
is specified.

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Patrick Delaunay <patrick.delaunay@st.com>