DO NOT COMMIT: Work around PI clash with one of the mcr20a-rf-driver registers

Author: bulislaw

cmsis/arm_math.h

@@ -345,8 +345,8 @@ extern "C"
 #define DELTA_Q31          (0x100)
 #define DELTA_Q15          0x5
 #define INDEX_MASK         0x0000003F
-#ifndef PI
-  #define PI               3.14159265358979f
+#ifndef _PI
+  #define _PI               3.14159265358979f
 #endif
 
   /**

docs/cmsis_rtx.md

@@ -33,6 +33,7 @@ Due to different use cases between mbed OS and CMSIS, we had to do some modifica
 Filename | Description |
 ---------|-------------|
 cmsis_compiler.h | Added IAR architecture macros |
+arm_math.h | Rename `PI` to `_PI` to avoid name clash with a register name in mcr20a-rf-driver |
 
 ### RTX