This flushes out the write to the system before anything continues.
The mips74k guide, chapter 3.3.3 (write gathering) notes that writes
can be buffered in FIFOs - even uncached ones - so we can't guarantee
the device has felt its effects. Now, since we're all lazy driver
authors and don't pepper read/write barriers everywhere, fake it here.
tested:
* mips74k - QCA9558 SoC (AP135 reference board)