boards: stm32n6570_dk: use ZEPHYR_REGION instead of SMH

Switch to usage of ZEPHYR_REGION for display and video
buffers instead of usage via the Shared-Multi-Heap API.
The ZEPHYR_REGION mechanism offer more flexibility than
the shared-multi-heap which is allocated on the whole
PSRAM.
If enabled, add allocation of the VENC internal buffers
from the PSRAM via the HEAP allocator.

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