clk: Allow clk_get_by_name() with NULL name

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

Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Link: https://lore.kernel.org/r/20230122000252.53642-1-samuel@sholland.org