dt: ar803x: document SmartEEE properties

The SmartEEE feature of Atheros AR803x PHYs can cause the link to
bounce. Add DT properties to allow SmartEEE to be disabled, and to
allow the Tw parameters for 100M and 1G links to be configured.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>