dt-bindings: gpio.txt: expand gpio-line-names recommendations

Provide more guidance to differentiate between recommendations for names of
lines which are hard-wired to on-board devices, versus recommendations for
names of lines which are connected to general-purpose pin headers.

Signed-off-by: Trevor Woerner <twoerner@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>