clk: add APIs to get (optional) clock by name without a device

Sometimes we may need get (optional) clock without a device,
that means use ofnode.
e.g. when the phy node has subnode, and there is no device created
for subnode, in this case, we need these new APIs to get subnode's
clock.

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Ryder Lee <ryder.lee@mediatek.com>