msm: smem: Remove use of MSM_SHARED_RAM_BASE define

Move to extracting smem region out of the device tree and using ioremap
to map it. This allows us to remove an user of iotable_init.

Change-Id: Idcabdbaf6961767989d0c7e5e6f3f2b6ef77bca3
Signed-off-by: Kumar Gala <galak@codeaurora.org>