dt-bindings: iio: light: isl29501: Rename bindings documentation file

Rename the bindings documentation file for Renesas ISL29501 Time-of-flight
sensor from isl29501.txt to renesas,isl29501.txt.

This is part of an ongoing effort to name bindings documentation files for
Renesas IP blocks consistently, in line with the compat strings they
document.

Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>