arm: mvebu: spl.c: Remove useless gd declaration

ddaa905 ("arm: mvebu: Add DM (driver model) support") removed the
assignment of the gd pointer, but kept the (now superfluous) declaration
of the gd pointer.

Remove this declaration.

Signed-off-by: Reinhard Pfau <pfau@gdsys.de>
Reviewed-by: Stefan Roese <sr@denx.de>
Signed-off-by: Stefan Roese <sr@denx.de>