ar71xx: fix RouterBOARD Power LED default state

ar71xx: fix RouterBOARD Power LED default state

RouterBOARD(s) bootloader actully turns Power LED off just before
it starts the kernel. So we need to set the LED default status to On
instead of Keep in order to keep LED on during kernel boot.

This change fixes Power LED off during the kernel boot on the RB91x and
SXT Lite boards.

Fixes: 6cad8ee0bd ("ar71xx: keep the RouterBOARD Power LED in On state")
CC: Mathias Kresin <dev@kresin.me>
Signed-off-by: Sergey Ryazanov <ryazanov.s.a@gmail.com>
(backported from 8baad26d01dd1f7c6b30a9eadb68596ee4b4d51f)