dt-bindings: clock: stm32mp1: describes clocks if "st,stm32mp1-rcc-secure"

In case of "st,stm32mp1-rcc-secure" (stm32mp1 clock driver with RCC
security support hardened), "clocks" and "clock-names" describe oscillators
and are required.

Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>
Reviewed-by: Rob Herring <robh@kernel.org>