msm: ep-pcie: Support 16.6 MHz Aux clock for PCIe Gen4 Controller

If the devicetree property for aux clock is present run the clock
at 16.6 MHz. Otherwise default aux clock value to 19.2 MHz.
Introduced for PCIe Gen4 support but can be used in the
future if controller needs 16.6 MHz clock support.

Change-Id: I8a4b3ab4cbd44938f1c9bf8fa91a791ba289ee10
Signed-off-by: Gauri Joshi <gaurjosh@codeaurora.org>