ARM: rmobile: Increase PHY auto-negotiation timeout to 20s

The ethernet PHY on all known Gen3 boards takes a while to come out
of reset, increase the auto-negotiation timeout to prevent it from
timing out in case the ethernet is used right after the board was
reset.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>