net: phy.h: Make PHY autonegotiation timeout configurable

The Marvell MV78460 eval board DB-78460-BP seems to need a longer
PHY autonegotiation timeout than the "standard" 4 seconds. So lets
make this timeout configurable. If not defined in the board config
header the original 4000ms is used.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Joe Hershberger <joe.hershberger@gmail.com>