clk: of-provider: look at parent if registered device has no provider info

It seems to be usual for MFD devices that the created 'clock sub-device'
do not have own DT node. The clock provider information is usually in the
main device node which is owned by the MFD device. Change the devm variant
of clk of-provider registration to check the parent device node if given
device has no own node or if the node does not contain the #clock-cells
property. In such case use the parent node if it contains the #clock-cells.

Signed-off-by: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
[sboyd@kernel.org: Add some comment in the code and pull out logic into
a single function to return the provider device_node pointer]
Signed-off-by: Stephen Boyd <sboyd@kernel.org>