ARM: dts: stm32: Add missing ETHCK clock to ethernet node on stm32mp1

Add missing 'eth-ck' clock to the ethernet node on stm32mp1. 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>
Cc: Alexandre Torgue <alexandre.torgue@st.com>
Cc: Christophe ROULLIER <christophe.roullier@st.com>
Cc: Maxime Coquelin <mcoquelin.stm32@gmail.com>
Cc: Patrice Chotard <patrice.chotard@st.com>
Cc: Patrick Delaunay <patrick.delaunay@st.com>
Cc: linux-stm32@st-md-mailman.stormreply.com
To: linux-arm-kernel@lists.infradead.org
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>