dts: arm: stm32h7 hsi clock requires hsi-div property

The compatible: "st,stm32h7-hsi-clock"  has HSI clock divider
required which is set to 1, by default, delevering 64MHz
in the stm32h7.dtsi and stm32h7rs.dtsi
(As done for stm32h5 and other series)

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