fbdev: Restrict FB_SH_MOBILE_LCDC to SuperH

Since commit f402f7a0

 ("staging: board: Remove Armadillo-800-EVA
board staging code"), there are no more users of the legacy SuperH
Mobile LCDC framebuffer driver on Renesas ARM platforms.  All former
users on these platforms have been converted to the SH-Mobile DRM
driver, using DT.

Suggested-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Helge Deller <deller@gmx.de>