common: bootm: Document fake bootm sub-command

Fake option is enabled only when CONFIG_TRACE is
enabled in common/bootm.c:do_boot_states().

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

common/cmd_bootm.c

@@ -185,6 +185,9 @@ static char bootm_help_text[] =
 	"\tcmdline - OS specific command line processing/setup\n"
 	"\tbdt     - OS specific bd_t processing\n"
 	"\tprep    - OS specific prep before relocation or go\n"
+#if defined(CONFIG_TRACE)
+	"\tfake    - OS specific fake start without go\n"
+#endif
 	"\tgo      - start OS";
 #endif