[MIPS] Add bcm1480 ZBus trace support, fix wait related bugs

Make ZBus tracing generic - moving it to a common direcotry under
arch/mips/sibyte, add bcm1480 support and fix some wait related
bugs (thanks to Ralf for assistance on that).

Signed-off-by: Mark Mason <mason@broadcom.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>