doc: dt-bindings: remove duplicates with dts/upstream

doc: dt-bindings: remove duplicates with dts/upstream

doc/device-tree-bindings/leds/leds-bcm6328.txt can be found at
dts/upstream/Bindings/leds/leds-bcm6328.yaml.

doc/device-tree-bindings/leds/leds-bcm6358.txt can be found at
dts/upstream/Bindings/leds/leds-bcm6358.txt.

doc/device-tree-bindings/leds/leds-gpio.txt can be found at
dts/upstream/Bindings/leds/leds-gpio.yaml.

doc/device-tree-bindings/leds/leds-lp5562.txt can be found at
dts/upstream/Bindings/leds/leds-lp55xx.yaml.

Only two LED dt-bindings are left in U-Boot: leds-bcm6858.txt and
leds-pwm.txt. The former is partially supported by
dts/upstream/Bindings/leds/leds-bcm63138.yaml but is lacking all
optional properties we have listed in "downstream" dt-binding in U-Boot.
However, there doesn't seem to exist any user of that compatible.
The latter is partially supported by
dts/upstream/Bindings/leds/leds-pwm.yaml but is missing the
u-boot,default-brightness property, which is used by
arch/arm/dts/rk3326-odroid-go2-u-boot.dtsi at the moment. The
default-brightness property is probably not what we want here as it
defaults to max-brightness if missing. I'm assuming we want a different
value for U-Boot (127) and the kernel (255 via max-brightness as a
default), which would prevent us from upstreaming this property, which
doesn't change the status quo, so let it be for now.

Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>