gpio: reintroduce devm_get_gpiod_from_child()

We need to keep this API around for the merge window to avoid
nasty build problems in the merges.

Cc: Lee Jones <lee.jones@linaro.org>
Suggested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>