soc: st: stm32: set default SMH attribute for LTDC/video buffers

The SMH attribute when using the XSPI PSRAM is set to EXTERNAL (2)
within the driver hence set default for both LTDC and video
buffer SMH attribute to 2 if all conditions are validated.

Signed-off-by: Alain Volmat <alain.volmat@foss.st.com>