arge: do a read-after-write on all arge register writes, not just MDIO writes.

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)

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