dt-bindings: clock: stm32mp1: adapt example for "st,stm32mp1-rcc-secure"

For "st,stm32mp1-rcc-secure" schema, clocks and clock-names entries are now
required properties.

Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220511092559.4952-1-alexandre.torgue@foss.st.com

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Signed-off-by: Arnd Bergmann <arnd@arndb.de>