msm: pcie: support PCIe loopback for SM8150

SM8150 PCIe cores have different offsets for iATU
registers. Update how iATU registers are accessed
and programmed for debugfs loopback.

Change-Id: Idd79de88171427b83342dd741670627610461de1
Signed-off-by: Tony Truong <truong@codeaurora.org>