msm: pcie: update PCIe internal AUX CLK from boolean to a programmable type

PCIe internal AUX clock frequency used to be either 1MHz or
19.2MHz. The devicetree entry was a boolean type to determine
which frequency to program in PCIe controller. Newer chipsets
may support other frequencies such as 16.6MHz. Update the
devicetree entry so that it accepts a decimal value for the
desired frequency to be programmed in PCIe controller.

Change-Id: Idd672bfb2908a15b9ee3e72654eb4d40a6d9b624
Signed-off-by: Tony Truong <truong@codeaurora.org>