samples: display: add stm32h7s78_dk board dedicated conf

Due to the large display of the STM32H7S78 DK, it is necessary
to have larger POOL_SIZE for display buffers.

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

Author: Alain Volmat

samples/drivers/display/boards/stm32h7s78_dk.conf

@@ -0,0 +1,6 @@
+# Copyright (c) 2025 ST Microelectronics
+# SPDX-License-Identifier: Apache-2.0
+
+# On STM32H7S78 DK board, display being larger, it requires
+# more memory to draw squares on the display (40K)
+CONFIG_HEAP_MEM_POOL_SIZE=65536