clk: qcom: hfpll: get parent clock names from DT

Allow accessing the parent clock name required for the driver
operation using the device tree node.

This permits extending the driver to other platforms without having to
modify its source code.

For backwards compatibility leave the previous value as default.

Co-developed-by: Niklas Cassel <niklas.cassel@linaro.org>
Signed-off-by: Niklas Cassel <niklas.cassel@linaro.org>
Signed-off-by: Jorge Ramirez-Ortiz <jorge.ramirez-ortiz@linaro.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Ondrej Kubik <ondrej.kubik@canonical.com>