ARM: imx_v6_v7_defconfig: Enable FB_MXC_MIPI_DSI_NORTHWEST

Enable 'FB_MXC_MIPI_DSI_NORTHWEST' config by default for imx6 and imx7
series which includes the Northwest Logic MIPI DSI controller.

Signed-off-by: Fancy Fang <chen.fang@nxp.com>