fbdev: sh_mobile_meram: Allocate ICBs automatically

Instead of manually specifying the ICBs to use in platform data,
allocate them automatically at runtime. The range of reserved ICBs (for
instance to be used through UIO), if any, is passed in the platform data
reserved_icbs field as a bitmask.

The MERAM registration function now returns a pointer to an opaque MERAM
object, which is passed to the update and unregistration functions.

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