Fix help for or1k builds

The help said cpu-type could be mor1kx, which is correct but you must
pass or1k to get mor1kx.  Fix the message to properly represent what
needs to be passed to the commandline.

litex/soc/integration/soc_core.py

@@ -384,7 +384,7 @@ class SoCCore(Module):
 
 def soc_core_args(parser):
     parser.add_argument("--cpu-type", default=None,
-                        help="select CPU: lm32, mor1kx, picorv32, vexriscv, minerva")
+                        help="select CPU: lm32, or1k, picorv32, vexriscv, minerva")
     parser.add_argument("--cpu-variant", default=None,
                         help="select CPU variant")
     parser.add_argument("--integrated-rom-size", default=None, type=int,