boards: st: stm32h750b_dk: increase FMC SDRAM clock period

Testing full SDRAM access on stm32h750b_disco showed instabilities
and corrupted accessed. Increasing the FMC SDRAM clock period fixes
the issue.

This change ensures stability of transactions with the SDRAM but
may be sub-optimized regarding performances. An alternate correction
would need further investigations in the FMC interface timings
and clocks configuration.

Signed-off-by: Etienne Carriere <etienne.carriere@st.com>