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 <arnd@arndb.de>
Signed-off-by: Peter Griffin <peter.griffin@linaro.org>
Cc: Jean-Christophe Plagniol-Villard <plagnioj@jcrosoft.com>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: linux-fbdev@vger.kernel.org
Cc: Wan ZongShun <mcuos.com@gmail.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>