fbdev/sbus: Add initializer macros and Kconfig tokens for SBUS support

Add macros to initialize struct fb_ops for drivers that support
framebuffers on Sparc's SBUS. Also add a Kconfig token that selects
the necessary helpers.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230806120926.5368-4-tzimmermann@suse.de