dwc3-msm: Add __iomem markings where needed

Sparse currently fails on this file with a "too many warnings"
message because these I/O read/write APIs don't have the proper
markings. Add them to silence sparse and catch future problems.

Change-Id: I9179dfe7dd0581e2b265fc9a7dad7b91fe013273
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>