rockchip: puma-rk3399: increase Ethernet PHY timeout to 30s

rockchip: puma-rk3399: increase Ethernet PHY timeout to 30s

On Puma v2.1 with a KSZ9031, the Ethernet PHY often doesn't detect the
link in the 4-second default timeout.

After some boot-loop test for about a thousand boots, 70% were below
10s, 90% below 15s and 100% below 30s. Let's play it safe and make it
30s so that hopefully all links that should be detected are.

Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>