am437x_evm: increase phy autoneg timeout

When AM437x EVM is connected to Gigabit switch, it takes
more time to finish auto-negotiation than on a 10/100 switch.

The default 4 second limit times-out more often than not. This is
observed when testing with a D-Link DGS-1008A desktop switch.

Increase the auto-negotiation time-out for AM437x EVM to handle
this case.

Signed-off-by: Sekhar Nori <nsekhar@ti.com>