boards: st: higher sysclock for the stm32h743/h753 nucleo boards

Configure the DTS to increase the sysclock to its max value
480MHz for the nucleo_h743zi and nucleo_h753zi boards
Max Hclock is 240MHz
The Pll source is HSE is 8MHz clock source from the STLink.

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