ARM720: help/usage updates

Deprecate the "pass an instruction opcode" flavor of cp15 access
in favor of the "arm mcr ..." and "arm mrc ..." commands, which
offer fewer ways to break things.

Use the same EBNF syntax in the code as for the user's guide.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>

src/target/arm720t.c

@@ -516,8 +516,10 @@ static const struct command_registration arm720t_exec_command_handlers[] = {
 		.name = "cp15",
 		.handler = arm720t_handle_cp15_command,
 		.mode = COMMAND_EXEC,
-		.usage = "<opcode> [value]",
-		.help = "display/modify cp15 register",
+		/* prefer using less error-prone "arm mcr" or "arm mrc" */
+		.help = "display/modify cp15 register using ARM opcode"
+			" (DEPRECATED)",
+		.usage = "instruction [value]",
 	},
 	COMMAND_REGISTRATION_DONE
 };