boards: st: n6: use hse as system source clock

Use HSE as system source clock and adjust the different clock parameters
to use the highest admissible frequency (according to datasheet DS14791):
- CPU clock: 800MHz
- NPU clock: 1000MHz
- AXI clock: 800MHz
- AHB clock: 200MHz
Remove the setting of APB prescalers since they are constants set to 1.

Signed-off-by: Guillaume Gautier <guillaume.gautier-ext@st.com>