Documentation: gpio: update the preferred method for using software node lookup

Documentation: gpio: update the preferred method for using software node lookup

In its current version, the manual for converting of board files from
using GPIO lookup tables to software nodes recommends leaving the
software nodes representing GPIO controllers as "free-floating", not
attached objects and relying on the matching of their names against the
GPIO controller's name. This is an abuse of the software node API and
makes it impossible to create fw_devlinks between GPIO suppliers and
consumers in this case. We want to remove this behavior from GPIOLIB and
to this end, work on converting all existing drivers to using "attached"
software nodes.

Except for a few corner-cases where board files define consumers
depending on GPIO controllers described in firmware - where we need to
reference a real firmware node from a software node - which requires a
more complex approach, most board files can easily be converted to using
propert firmware node lookup.

Update the documentation to recommend attaching the GPIO chip's software
nodes to the actual platform devices and show how to do it.

Reviewed-by: Linus Walleij <linusw@kernel.org>
Link: https://patch.msgid.link/20260403-doc-gpio-swnodes-v2-1-c705f5897b80@oss.qualcomm.com
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>