UPSTREAM: dm: backlight: Add a driver for GPIO backlight

Add a driver for GPIO backlights.
It understands the standard device tree binding.
It can be used with simple-panel when PWM is not necessary

UPSTREAM: dm: backlight: Add a driver for GPIO backlight

Add a driver for GPIO backlights.
It understands the standard device tree binding.
It can be used with simple-panel when PWM is not necessary.

Change-Id: Ia4160d1dfbe53b3f6211f3d321c33b8b5d0be6d0
Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
(cherry picked from commit 5b6a6a91c870949e21b7fd3865582ea6a32ea922)

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