dt, sgtl5000: clean up audio clock handling by passing a real fixed clock

Passing clock-frequency right in the codec node is horrible legacy evil, so
quit doing it - pass a real fixed clock instead.

Actually this is kind of weird as fixed-rate clock support doesn't seem to
have any enable/disable support, but we can definitely enable/disable it..
dt, sgtl5000: clean up the mclk node by removing unused gpio property

.. but keep it around in a new node for a clock which doesn't exist in the
Linux kernel yet (since there's a couple hundred lines of code needed to
be written to manage it properly). Give it a crazy compatible so that it
doesn't ever get probed for now. This node is NOT to be published!!!

Also clean up the eSDHC GPIO comments (no point repeating the same info)
while we're here