hack: librem5_defconfig: Set CONFIG_BACKLIGHT_LED back to module

The screen doesn't turn on at all with BACKLIGHT_LED built-in and
simple_ondemand devfreq patchset at the moment.

arch/arm64/configs/librem5_defconfig

@@ -553,7 +553,7 @@ CONFIG_DRM_MXSFB=y
 CONFIG_FB_MODE_HELPERS=y
 CONFIG_BACKLIGHT_CLASS_DEVICE=y
 CONFIG_BACKLIGHT_PWM=y
-CONFIG_BACKLIGHT_LED=y
+CONFIG_BACKLIGHT_LED=m
 CONFIG_LOGO=y
 # CONFIG_LOGO_LINUX_MONO is not set
 # CONFIG_LOGO_LINUX_VGA16 is not set