clk: Use dummy clk_get_by_* functions when CONFIG_CLK is disabled

The implementations of clk_get_by_index & clk_get_by_name are only
available when CONFIG_CLK is enabled. Provide the dummies when this is
not the case in order to avoid build failures.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Reviewed-by: Simon Glass <sjg@chromium.org>