doc: remove u-boot,boot-led and u-boot,error-led from "binding"

doc: remove u-boot,boot-led and u-boot,error-led from "binding"

We're aiming to reduce the amount of U-Boot-specific and out-of-spec
Device Tree additions.

Those two properties haven't been doing anything for a long time
already, except when read by board files manually. This is still the
case for STM32 boards but those will be migrated in the near future
according to their maintainer. In any case, let's not encourage people
to add either of these properties to new or existing Device Trees and
remove it from the bindings.

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