doc/xtensa: update supported architecture list

- Xtensa LX8 is fully supported in addition to prior LX and NX cores.

Change-Id: I2f3f0a21ce1518b3ced6d241f0ab84c65af64423
Signed-off-by: Ian Thompson <ianst@cadence.com>

doc/openocd.texi

@@ -11611,8 +11611,8 @@ on @url{https://www.nxp.com}.
 @subsection Xtensa Configuration Commands
 
 @deffn {Config Command} {xtensa xtdef} (@option{LX}|@option{NX})
-Configure the Xtensa target architecture. Currently, Xtensa support is limited
-to LX6, LX7, and NX cores.
+Configure the Xtensa target architecture to LX or NX. Currently, Xtensa LX support
+is limited to LX6 and newer cores.
 @end deffn
 
 @deffn {Config Command} {xtensa xtopt} option value