msm: cache_erp: remove use of MSM_IMEM_BASE

Move to getting address for cache_erp 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: Id1e3d1964fe07654e1751131ee49389b79e5d8b6
Signed-off-by: Kumar Gala <galak@codeaurora.org>