backlight: lcd: Add missing kerneldoc entry for 'struct device parent'

This has been missing since the conversion to 'struct device' in 2007.

Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Cc: Jamey Hicks <jamey.hicks@hp.com>
Cc: Andrew Zabolotny <zap@homelink.ru>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>