Update LangRef for the new constraint code for 128-bit values in NVPTX.

Author: Chengjunp

llvm/docs/LangRef.rst

@@ -5381,6 +5381,7 @@ NVPTX:
 - ``c`` or ``h``: A 16-bit integer register.
 - ``r``: A 32-bit integer register.
 - ``l`` or ``N``: A 64-bit integer register.
+- ``q``: A 128-bit integer register.
 - ``f``: A 32-bit float register.
 - ``d``: A 64-bit float register.