ARM: at91: remove phy fixup for sama5d3xek boards

These board specific delays are now configured through micrel's specific
DT bindings (see Documentation/devicetree/bindings/net/micrel-ksz9021.txt).

Remove this phy fixup registration from sama5 DT machine file to keep it
as generic as possible.

Signed-off-by: Boris BREZILLON <boris.brezillon@free-electrons.com>
Tested-by: Bo Shen <voice.shen@atmel.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>