ARM: imx_v6_v7_defconfig: Restore CONFIG_BACKLIGHT_CLASS_DEVICE

Since commit bfac19e2 ("fbdev: mx3fb: Remove the driver") backlight
is no longer functional.

The fbdev mx3fb driver used to automatically select
CONFIG_BACKLIGHT_CLASS_DEVICE.

Now that the mx3fb driver has been removed, enable the
CONFIG_BACKLIGHT_CLASS_DEVICE option so that backlight can still work
by default.

Tested on a imx6dl-sabresd board.

Cc: stable@vger.kernel.org
Fixes: bfac19e2

 ("fbdev: mx3fb: Remove the driver")
Signed-off-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Francesco Dolcini <francesco.dolcini@toradex.com>
Tested-by: Francesco Dolcini <francesco.dolcini@toradex.com> # Toradex Colibri iMX7
Signed-off-by: Shawn Guo <shawnguo@kernel.org>