msm: restart: remove {emergency_}download_mode use of MSM_IMEM_BASE

Move to getting address for {emergency_}download_mode 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: Ibe6d9238dd3c38c8037d157f82e0512a8bad5772
Signed-off-by: Kumar Gala <galak@codeaurora.org>