msm: restart: remove use restart_reason use of MSM_IMEM_BASE

Move to getting address for restart_reason 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: I308f45fe776a05638e2e1b960bd038c6826d7903
Signed-off-by: Kumar Gala <galak@codeaurora.org>