ARM: mxs_defconfig: Enable the framebuffer

Currently, when booting Linux on a imx28-evk board there is
no display activity.

Enable CONFIG_FB which is nowadays required for CONFIG_DRM_PANEL_LVDS,
CONFIG_DRM_PANEL_SIMPLE, CONFIG_DRM_PANEL_SEIKO_43WVF1G,
CONFIG_FB_MODE_HELPERS, CONFIG_BACKLIGHT_PWM, CONFIG_BACKLIGHT_GPIO,
CONFIG_FRAMEBUFFER_CONSOLE, CONFIG_LOGO, CONFIG_FONTS, CONFIG_FONT_8x8
and CONFIG_FONT_8x16.

Based on commit c5446748 ("ARM: imx_v6_v7_defconfig: enable fb").

Fixes: f611b1e7

 ("drm: Avoid circular dependencies for CONFIG_FB")
Signed-off-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>