fbdev: sh_mobile_meram: Add struct sh_mobile_meram_icb

The new structure stores ICB parameters for ICBs.

Instead of modifying the struct sh_mobile_meram_cfg instances passed by
callers, store the ICB parameters internally and make the public API
take const pointers to sh_mobile_meram_cfg.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>