msm: pcie: add mhi register space as PCIe RC resource

There are MHI registers which PCIe RC can use for debug
purposes such as dumping PCIe link ASPM stats. Add mhi
register space as a PCIe RC resource so that RC driver can
access MHI space.

Change-Id: I4392219b4f2e7007f5f263e4a9285bf630b2e71b
Signed-off-by: Tony Truong <truong@codeaurora.org>