Blackfin: bf518f-ezbrd: reset ethernet PHY during init

We don't know what state the ethernet PHY is in when starting up, so make
sure we set it to a sane state.  This fixes troubles seen when Linux boots
up, configures the PHY is a non-default state, and then the system reboots
into U-Boot which previously expected a reset state only.

Signed-off-by: Graf Yang <graf.yang@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>