Update FLASH_SIZE backup value

By default, FLASH_SIZE should be read from HW.
In case this is not the case, we define it here, as the size of FLASH
that is available to the application running on M4.

Author: LMESTM

targets/TARGET_STM/TARGET_STM32WB/TARGET_STM32WB55xG/device/flash_data.h

@@ -37,8 +37,8 @@
 #undef FLASH_SIZE
 #endif
 
-// Only the first 128 pages are accessible when security is enabled
-#define FLASH_SIZE    ((uint32_t)0x80000) // 128 pages x 4 Kbytes = 512 Kbytes
+// Only the first the application side is accessible.
+#define FLASH_SIZE    ((uint32_t)0xC0000) // 768 Kbytes
 
 #endif
 #endif