boards: st: stm32f746g_disco: Fix sdram available size

According to UM1907 Rev 6 section 5.13 "SDRAM Memory" [1], the
available SDRAM size is only 8MByte since only the lowest 64Mbit are
accessible despite the SDRAM capacity is 16MByte (128MBit).

Link: https://www.st.com/resource/en/user_manual/um1907-discovery-kit-for-stm32f7-series-with-stm32f746ng-mcu-stmicroelectronics.pdf [1]
Signed-off-by: Etienne Carriere <[email protected]>

Author: etienne-lms

boards/st/stm32f746g_disco/stm32f746g_disco.dts

@@ -54,7 +54,7 @@
 	sdram1: sdram@c0000000 {
 		compatible = "zephyr,memory-region", "mmio-sram";
 		device_type = "memory";
-		reg = <0xc0000000 DT_SIZE_M(16)>;
+		reg = <0xc0000000 DT_SIZE_M(8)>;
 		zephyr,memory-region = "SDRAM1";
 		zephyr,memory-attr = <DT_MEM_ARM(ATTR_MPU_RAM)>;
 	};