arm64: dts: qcom: sdm632: motorola-ocean: Add brightness levels

[ultra-azu]

This assigns 16 levels of brightness for backlight, and sets level 9 as default. In my rather crude test of using phosh's brightness slider, it seemed visually the same as without the levels.

[ultra-azu]

The issue I have is that dmesg reports this:

[  226.815702] panel-tianma-622-v0 1a94000.dsi.0: Failed to set display off: -22
[  226.815733] panel-tianma-622-v0 1a94000.dsi.0: Failed to un-initialize panel: -22

However I think it's very likely a false positive and in Matrix one person having the same issue. So I'll just leave the PR like it is

[z3ntu]

Please add Signed-off-by. Otherwise looks good

[ultra-azu]

Done