ramips: fix hg255d LED status support

Use the green power LED for boot status indication.

Source: https://my.oschina.net/osbin/blog/278782 Para 3

Signed-off-by: David Yang <[email protected]>

target/linux/ramips/base-files/etc/diag.sh

@@ -24,6 +24,7 @@ get_status_led() {
 	ex2700|\
 	ex3700|\
 	fonera20n|\
+	hg255d|\
 	kn|\
 	kn_rc|\
 	kn_rf|\