dt-bindings:iio:light:renesas,isl29501: Move to trivial devices.

This binding is so simple there is no obvious advantage in maintaining
a separate binding doc file for it. As such, move it to trivial-devices.yaml

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Simon Horman <horms+renesas@verge.net.au>
Link: https://lore.kernel.org/r/20201031182922.743153-6-jic23@kernel.org