dt: bindings: Add bindings for Cirrus Logic CS89x0 ethernet chip

Add device tree binding documentation details for Cirrus Logic
CS8900/CS8920 ethernet chip.

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>