drivers: memc: stm32 psram: Fix XSPI configuration for performance

Correct XSPI configuration in order to improve PSRAM access on the
STM32N6 discovery board.
Ideally, this should be defined by device tree, but I'm fixing
the only user for now.

Signed-off-by: Erwan Gouriou <erwan.gouriou@st.com>