dts: arm: st: stm32h7 with dual core have flash clock enable bit

Define the "clocks" property, for the flash "st,stm32h7-flash-controller"
node, only for the stm32H7 dual-core devices
which have the RCC bit 8 present in their RCC AHB3 register.

Signed-off-by: Francois Ramu <francois.ramu@st.com>