ARM: dts: stm32mp1: explicit clock reference needed by RCC clock driver

ARM: dts: stm32mp1: explicit clock reference needed by RCC clock driver

Define in the RCC clock provider node which root clocks the driver
depends on. These are root oscillators, which may be present or
not, upon FDT content.

This update binding is introduced in Linux kernel device tree by patch
"ARM: dts: stm32: move clocks/resets to SCMI resources for stm32mp15"

This patch is a preliminary step for SCMI support of stm32mp15
boards with trusted boot chain, based on TF-A or OP-TEE.

Signed-off-by: Etienne Carriere <etienne.carriere@st.com>
Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>