ARM: stm32: Increase PHY auto-negotiation timeout to 20s on DHSOM

ARM: stm32: Increase PHY auto-negotiation timeout to 20s on DHSOM

The Micrel PHYs on known DHSOM based boards take 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 <marex@denx.de>
Cc: Patrice Chotard <patrice.chotard@foss.st.com>
Cc: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>