[M2351] Fix serial sync error in SPDMC test

In M2351 SPDMC (SMCC) test, it boots from secure code to non-secure bootloader, and finally
to non-secure test code itself. The boot sequence will takes longer than usual. In test
initiation, host will send sync signal 1s after reset device. But due to the longer boot
sequence, test code in device can miss the signal. We enlarge the reset idle time to fix
the issue.

Author: ccli8

targets/targets.json

@@ -8104,6 +8104,7 @@
         "device_name": "M2351KIAAEES",
         "bootloader_supported": true,
         "tickless-from-us-ticker": true,
+        "forced_reset_timeout": 3,
         "mbed_rom_start"    : "0x10040000",
         "mbed_rom_size"     : "0x40000",
         "mbed_ram_start"    : "0x30008000",