video: Kconfig: Allow more broad selection of the imxfb framebuffer driver.

Without that patch, a user can't select the imxfb driver when the i.MX25
and/or the i.MX27 device tree board are selected and that no boards that
selects IMX_HAVE_PLATFORM_IMX_FB are compiled in.

Cc: Eric Bénard <eric@eukrea.com>
Cc: Jean-Christophe Plagniol-Villard <plagnioj@jcrosoft.com>
Cc: Sascha Hauer <kernel@pengutronix.de>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-fbdev@vger.kernel.org
Signed-off-by: Denis Carikli <denis@eukrea.com>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>