Address comments.

- Add `unused` case
- Drop `undefined` case
- Made 3~1023 as reserved for future standard defined platform register.

NOTE:
GP as SCS will defined in separated PR.

riscv-elf.adoc

@@ -1173,8 +1173,8 @@ global pointer relaxation, reserved platform register or a temporary register.
 [horizontal]
 0:: This object use gp as global pointer relaxation.
 1:: This object use gp as temporary register.
-127:: This object use gp as platform register for undefined purpose.
-128~1023:: Reserved for future standard defined platform register.
+2:: This object does not use gp register as global pointer relaxation or temporary register.
+3~1023:: Reserved for future standard defined platform register.
 1024~2047:: Reserved for nonstandard defined platform register.
 
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