dt-bindings: arm: msm: update documentation for PCIe internal AUX CLK

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: I5818f5973ddbbce4268eafb6e99202daeba4234a