fbdev: Add fb_ops init macros for framebuffers in DMA-able memory

Add initializer macros for struct fb_ops for framebuffers in DMA-able
memory areas. Also add a corresponding Kconfig token. As of now, this
is equivalent to system framebuffers and mostly useful for labeling
drivers correctly.

A later patch may add a generic DMA-specific mmap operation. Linux
offers a number of dma_mmap_*() helpers for different use cases.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Acked-by: Maxime Ripard <mripard@kernel.org>
Cc: Helge Deller <deller@gmx.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20230707083422.18691-2-tzimmermann@suse.de