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.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
brcm47xx: fix brcm47xx.legacy build
brcm47xx: image: update list of Netgear WNR* firmwares
ar7: remove 972-cpmac_fixup patch

This patch is causing more harm than good on most AR7 routers out there,
better have no manageable switch rather than no ethernet connection, at
least for now.
Fixes #16523, #5927