sh: ecovec24: Remove mmcif .down_pwr() callback

The callback isn't used by the mmcif driver, don't initialize it in
board code.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Chris Ball <cjb@laptop.org>