ARM: imx_v6_v7_defconfig: Enable backlight gpio support.

The eukrea mbimxsd51 has a gpio backlight for its
  LCD display, so we turn that driver on.

Cc: Sascha Hauer <kernel@pengutronix.de>
Cc: linux-arm-kernel@lists.infradead.org
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Cc: Eric Bénard <eric@eukrea.com>
Signed-off-by: Denis Carikli <denis@eukrea.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>