msm: pcie: Add VReg voltage and clock frequency

Retrive voltage of regulator and clock frequency from device tree.
Most chipsets use the default values. If a chipset has particular
values specified in device tree node, then override the default
values with those values.

Change-Id: I1d21467cf736f711e97300b32674a299520d38f2
Signed-off-by: Yan He <yanhe@codeaurora.org>