arm: am335x: DXR2: Move unconditional LAN9303 reset into command

The switch HW reset results in a disconnection of the switch port daisy-
chain for a few seconds. This is not desired in the normal field use
case. So lets remove this switch reset from the normal bootup sequence
and move it into a board specific command. This way it can be executed
in the development environment when really needed.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Roger Meier <r.meier@siemens.com>
Cc: Lukas Stockmann <lukas.stockmann@siemens.com>
Cc: Tom Rini <trini@ti.com>