brcm47xx: fix reboot problem on BCM4705/BCM4785

brcm47xx: fix reboot problem on BCM4705/BCM4785

This adds some code based on code from the Broadcom GPL tar to fix the
reboot problems on BCM4705/BCM4785. I tried rebooting my device for ~10
times and have never seen a problem. This reverts the changes in the
previous commit and adds the real fix as suggested by Rafał.

Setting bit 22 in Reg 22, sel 4 puts the BIU (Bus Interface Unit) into
async mode.

backport of: r42083

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
git-svn-id: svn://svn.openwrt.org/openwrt/branches/barrier_breaker@42086 3c298f89-4303-0410-b956-a3cf2f4a3e73