pci: qcom: Add support to external phy reference clk.

This patch adds support to configuration which use external phy reference clk,
this option is now available via dt using "qcom,external-phy-refclk" property.

Without this option the driver will always configure the phy to use
internal clk which will not work for SOCs like APQ8064 which uses
external phy reference clock on IFC6410 board.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>