Fix page size in flash IAP

In Mbed OS, page size is program unit, which is different than FMC definition.
After fixing page size, we can pass NVSTORE test (mbed-os-features-nvstore-tests-nvstore-functionality).

Author: ccli8

targets/TARGET_NUVOTON/TARGET_M451/flash_api.c

@@ -65,7 +65,8 @@ static const sector_info_t sectors_info[] = {
 };
 
 static const flash_target_config_t flash_target_config = {
-    .page_size  = 0x800,    // 2 KB
+    .page_size  = 4,        // 4 bytes
+                            // Here page_size is program unit, which is different than FMC definition.
     .flash_start = 0x0,
     .flash_size = 0x40000,  // 256 KB
     .sectors = sectors_info,

targets/TARGET_NUVOTON/TARGET_M480/flash_api.c

@@ -69,7 +69,8 @@ static const sector_info_t sectors_info[] = {
 };
 
 static const flash_target_config_t flash_target_config = {
-    .page_size  = 0x200,    // 512 bytes
+    .page_size  = 4,        // 4 bytes
+                            // Here page_size is program unit, which is different than FMC definition.
     .flash_start = 0x0,
     .flash_size = 0x80000,  // 512 KB
     .sectors = sectors_info,

targets/TARGET_NUVOTON/TARGET_NUC472/flash_api.c

@@ -67,7 +67,8 @@ static const sector_info_t sectors_info[] = {
 };
 
 static const flash_target_config_t flash_target_config = {
-    .page_size  = 0x800,    // 2 KB
+    .page_size  = 4,        // 4 bytes
+                            // Here page_size is program unit, which is different than FMC definition.
     .flash_start = 0x0,
     .flash_size = 0x80000,  // 512 KB
     .sectors = sectors_info,