ARM: dts: rockchip: drop veyron's nonstandard 'backlight-boot-off'

This was used out-of-tree as a hack for resolving issues where some
systems expect the backlight to turn on automatically at boot, while
others expect to manage the backlight status via a DRM/panel driver.
Those issues have since been fixed upstream in pwm_bl.c without device
tree hacks, and so this un-documented property should no longer be
useful.

Signed-off-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>