ARM_MUSCA_B1: default baud rate to 115200

The TF-M secure binary has a fixed baud rate of 115200. Having a
different baud rate on the non-secure side results in broken serial
outputs.

Author: LDong-Arm

platform/mbed_lib.json

@@ -235,6 +235,10 @@
             "crash-capture-enabled": true,
             "fatal-error-auto-reboot-enabled": true
         },
+        "ARM_MUSCA_B1": {
+            "stdio-convert-newlines": true,
+            "stdio-baud-rate": 115200
+        },
         "ARM_MUSCA_S1": {
             "stdio-convert-newlines": true,
             "stdio-baud-rate": 115200