Add basic logging for DPCD access. Does not print
by default.

Currently only prints first byte of the data accessed.

Technical debt: Need to make it so that the entire
data block accessed is printed. Also need to log
address space that's not DPCD.
Signed-off-by: Eric Yang <Eric.Yang2@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>