phy: qcom-qmp-pcie: restructure PHY creation

In preparation for supporting devicetree bindings which do not use a
child node, move the PHY creation to probe() proper and parse the serdes
resource in what is now the legacy devicetree helper.

Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Johan Hovold <johan+linaro@kernel.org>