net: phy: Support overriding Auto Negotiation timeout with env variable

net: phy: Support overriding Auto Negotiation timeout with env variable

The Auto Negotiation procedure between two Ethernet PHYs consists of
determining the best commonly supported parameters among Speed,
Duplex Mode and Flow Control.

The time taken for this procedure is not only dependent on the local
PHY used, but also on the link-partner PHY.

While a timeout can be specified in the form of a "CONFIG" on the basis
of the local PHY present on the device, since the timeout also depends
on the link-partner PHY, it might be necessary to modify the timeout.

To avoid rebuilding the bootloader for a given device, just because it
may be connected to various link-partner PHYs, each with a different
timeout, introduce an environment variable named "phy_aneg_timeout" and
override "CONFIG_PHY_ANEG_TIMEOUT" with "phy_aneg_timeout".

Signed-off-by: Siddharth Vadapalli <s-vadapalli@ti.com>
Acked-by: Jerome Forissier <jerome.forissier@linaro.org>
[jf: add missing #include <env.h>]
Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>