gpio: allow passing NULL to gpio_request_by_line_name() to search all gpio controllers

The API is more convenient to use if one doesn't have to know upfront
which gpio controller has a line with the name one is searching for,
and arrange to look that device up somehow. Or implement this loop
oneself.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Reviewed-by: Simon Glass <sjg@chromium.org>