fbdev: Provide I/O-memory helpers as module

Provide helpers for accessing I/O memory in a helper module. The fbdev
core uses these helpers, so select the module unconditionally for fbdev.
Drivers will later be able to select the module individually and the
helpers will become optional.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230927074722.6197-2-tzimmermann@suse.de