msm: msm_memory_dump: remove use of MSM_IMEM_BASE

Move to getting address for msm_memory_dump region of IMEM from the device
tree and mapping via ioremap.  This removes a usage of MSM_IMEM_BASE which
will allow removal of the static IO mappings.

Change-Id: I99ec8a87c782b1d30e22606bf849b538bb95815e
Signed-off-by: Kumar Gala <galak@codeaurora.org>