reset: Allow reset_get_by_name() with NULL name

This allows devm_reset_control_get(dev, NULL) to work and get the first
reset control, which is common in code ported from Linux.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Link: https://lore.kernel.org/r/20230122000252.53642-2-samuel@sholland.org