boards: st: stm32h7s78_dk: Fix ram size in board yaml file

Actual available internal RAM size was wrong, fix it.

Signed-off-by: Erwan Gouriou <[email protected]>

Author: erwango

boards/st/stm32h7s78_dk/stm32h7s78_dk.yaml

@@ -4,7 +4,7 @@ type: mcu
 arch: arm
 toolchain:
   - zephyr
-ram: 640
+ram: 128
 flash: 64
 supported:
   - arduino_gpio