MIPS: BCM47XX: Fix reboot problem on BCM4705/BCM4785

MIPS: BCM47XX: 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.

The previous commit was 95403f13e0a64af5fccb02643cfa05494c2593ce [MIPS:
BCM47XX: make reboot more relaiable]

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Cc: jogo@openwrt.org
Cc: zajec5@gmail.com
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/7545/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>