video/nuc900: allow modular build

The frame buffer core may be a loadable module, and in this
case, the nuc900 driver cannot be built-in. Turning it into
a tristate option lets Kconfig work out the dependency and
avoid broken builds.

Signed-off-by: Arnd Bergmann <[e-mail]>
Signed-off-by: Peter Griffin <[e-mail]>
Cc: Jean-Christophe Plagniol-Villard <[e-mail]>
Cc: Tomi Valkeinen <[e-mail]>
Cc: [e-mail]
Cc: Wan ZongShun <[e-mail]>
Signed-off-by: Tomi Valkeinen <[e-mail]>

drivers/video/fbdev/Kconfig

···
 	  through sysfs
 
 config FB_NUC900
-        bool "NUC900 LCD framebuffer support"
+        tristate "NUC900 LCD framebuffer support"
         depends on FB && ARCH_W90X900
         select FB_CFB_FILLRECT
         select FB_CFB_COPYAREA