Allow accessing the parent clock names required for the driver operation
by using the device tree 'clock-names' property, while falling back to
the previous method of using names in the global name space.

This permits extending the driver to other platforms without having to
modify its source code.
Co-developed-by: Jorge Ramirez-Ortiz <jorge.ramirez-ortiz@linaro.org>
Signed-off-by: Jorge Ramirez-Ortiz <jorge.ramirez-ortiz@linaro.org>
Signed-off-by: Niklas Cassel <niklas.cassel@linaro.org>
Link: https://lkml.kernel.org/r/20200103111429.1347-1-nks@flawful.orgTested-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>