Fix LCD backlight issue for droid4

There was a bit of an integration glitch with the LED backlight series.
The LED related parts got merged into v5.6-rc1, but the actual backlight
driver got left out.

This caused an issue on at least droid4 where the LCD backlight can not
yet be enabled automatically. And the LCD backlight can no longer be
enabled manually either via sysfs.

The integration glitch happened because some pending comments from me.
There was some confusion on which device tree property we should use for
the default brightness property.

After discussing how to fix this on the mailing lists, we came to the
conclusion that it's best to fix this issue properly by adding the
missing driver.

The other solutions would mean backpedaling and try to come up with some
temporary solution that really does not solve the issue for users. The
patch for led_bl.c has been around for quite a while and tested by many
users and was assumed to be merged as part of the LED backlight series.

For the brightness property to use, we ended up using the more common
"default-brightness-level" rather than "default-brightness" used by
some backlight drivers.
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