arm64: dts: rockchip: Use default brightness table for rk3399-gru

After commit 88ba95be

 ("backlight: pwm_bl: Compute brightness of LED
linearly to human eye") the pwm_bl driver is able to calculate a default
brightness table. The calculated table for this PWM will have more
granularity and will be adjusted to change the brightness linearly to
the human eye. Use that table instead of have a DT-defined table with
less granularity.

Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>