Update FLASH_SIZE backup value

By default, FLASH_SIZE should be read from HW.
n 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)0xCB000) // 812 Kbytes
 
 #endif
 #endif