PHY: micrel.c: add support for KSZ9031

Add support for Micrel PHY KSZ9031 in phylib,
including small rework for KSZ9021 to avoid
code duplication

Signed-off-by: David Andrey <david.andrey@netmodule.com>
Cc: Troy Kisky <troy.kisky@boundarydevices.com>
Cc: Joe Herschberger <joe.hershberger@gmail.com>
Cc: Andy Fleming <afleming@freescale.com>
Acked-by: Stefan Roese <sr@denx.de>