net: phy: at803x: add support for configuring SmartEEE

SmartEEE for the atheros phy was deemed buggy by Freescale and commits
were added to disable it for their boards.

In initial testing, SolidRun found that the default settings were
causing disconnects but by increasing the Tw buffer time we could allow
enough time for all parts of the link to come out of a low power state
and function properly without causing a disconnect. This allows us to
have functional power savings of between 300 and 400mW, rather than
disabling the feature altogether.

This commit adds support for disabling SmartEEE and configuring the Tw
parameters for 1G and 100M speeds.

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