Describe optional external ethernet RX clock in the DT binding
to fix dtbs_check warnings like:

arch/arm/boot/dts/stm32mp153c-dhcom-drc02.dt.yaml: rcc@50000000: 'assigned-clock-parents', 'assigned-clock-rates', 'assigned-clocks', 'clock-names', 'clocks' do not match any of the regexes: 'pinctrl-[0-9]+'
Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Alexandre Torgue <alexandre.torgue@foss.st.com>
Cc: Gabriel Fernandez <gabriel.fernandez@foss.st.com>
Cc: Rob Herring <robh+dt@kernel.org>
To: devicetree@vger.kernel.org
Acked-by: Gabriel Fernandez <gabriel.fernandez@foss.st.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220410220514.21779-1-marex@denx.de