dm: gpio: Fix gpio_get_list_count failing with livetree

of_parse_phandle_with_args (called by dev_read_phandle_with_args) does not
support getting the length of a phandle list by using the index -1.
Instead, use dev_count_phandle_with_args which supports exactly this
use-case.

Fixes: 85582171 ("gpio: Convert to use APIs which support live DT")

Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Reviewed-by: Simon Glass <sjg@chromium.org>