Prevent warnings during compilations for MAX32620C

In Mbed OS 5.12.0 mbed_toolchain.py has been introduced, that automatically defines the macro TARGET_NAME (on line 241). This leads to a high number of generating the following warning: 'TARGET_NAME' macro redefined while compiling code for MAX32620C on Mbed OS 5.12 and above. This patch fixes it, while it keeps the definition in place for lower versions that lack mbed_toolchain.py

Author: idea--list

targets/TARGET_Maxim/TARGET_MAX32620C/device/mxc_device.h

@@ -46,7 +46,9 @@
 // Create a string definition for the TARGET
 #define STRING_ARG(arg) #arg
 #define STRING_NAME(name) STRING_ARG(name)
+#if MBED_VERSION && MBED_VERSION < 51200
 #define TARGET_NAME STRING_NAME(TARGET)
+#endif
 
 // Define which revisions of the IP we are using
 #ifndef TARGET_REV