arm: am335x: DXR2: Reset SMSC LAN9303 switch via GPIO upon bootup

Since the switch may be re-configured for VLAN usage in Linux (or any
other OS), lets reset the switch to its default register values upon
power-up. Otherwise network might not be available in U-Boot.

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