[PATCH master 1/2] clk: implement clk_get_optional helper

[Marco Felsch]

On 23-09-04, Ahmad Fatoum wrote:
> Lack of clock can be ok at times and thus the clk API accepts NULL
> arguments and treats them as no-op.
> 
> Linux provides a clk_get_optional function to simplify code with such
> optional code, so let's provide an equivalent for barebox.
> 
> Signed-off-by: Ahmad Fatoum <a.fatoum at pengutronix.de>

Reviewed-by: Marco Felsch <m.felsch at pengutronix.de>