arge_mdio: add explicit read barriers for MDIO_READs.

The mips74k programmers guide notes that reads can be re-ordered, even
uncached ones, so we need an explicit SYNC between them.

Yes, this is a case of a driver author actively doing a bus barrier
operation.

This ends up being necessary when the mips74k core is run in write-back
mode rather than write-through mode.  That's coming in an upcoming
commit.

Tested:

* mips74k, QCA9558 SoC (AP135 reference board), arge<->arge interface
  routing traffic tests.

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