drivers: memc: compute prescaler automatically for stm32 xspi

For STM32 XSPI PSRAM driver, compute and set the prescaler automatically
according to the kernel clock and the max frequency of the PSRAM.
Copied from what is done in the STM32 XSPI Flash driver.

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