fbdev/efifb: Add struct efifb_par for driver data

The efifb_par structure holds the palette for efifb. It will also
be useful for storing the device's aperture range.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221219160516.23436-11-tzimmermann@suse.de