net: dsa: microchip: rename ksz8 series files

The first KSZ8 series implementation was done for a KSZ8795 device but
since several other KSZ8 devices have been added. Rename these files
to adhere to the ksz8 naming convention as already used in most
functions and the existing ksz8.h; add an explanatory note.

Signed-off-by: Pieter Van Trappen <pieter.van.trappen@cern.ch>
Acked-by: Arun Ramadoss <arun.ramadoss@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>