clk: Add clk_get_by_name_optional

This adds a helper function for clk_get_by_name in cases where the clock is
optional. Hopefully this helps point driver writers in the right direction.
Also convert some existing users.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Link: https://lore.kernel.org/r/20220115205247.566210-2-seanga2@gmail.com