dt-bindings: net: dsa: convert ksz bindings document to yaml

Convert the bindings document for Microchip KSZ Series Ethernet switches
from txt to yaml. Removed spi-cpha and spi-cpol flags is this should be
handled by the device driver.

Signed-off-by: Christian Eggers <ceggers@arri.de>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>