brcm63xx: fix crash on 6358/6368 when booting from thread 1

It looks like RAC flushes cause random corruption(?) when the
second thread is set as default.

Fixes #20160.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@46563 3c298f89-4303-0410-b956-a3cf2f4a3e73