msm: boot_stats: remove use of MSM_IMEM_BASE

Move to getting address for boot_stats 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.

Additionally move to using io accessors since we are accessing IO memory
instead of direct pointer manipulation.

Change-Id: Ib3d3494612b37e7ee9903d270c80ea929d6a7435
Signed-off-by: Kumar Gala <galak@codeaurora.org>