soc: qcom: smem: Move RPM message ram out of smem DT node

SMEM is a software construct built on top of a DDR reserved region
and sometimes a device memory region called RPM message ram. Having
the RPM message ram in the smem DT node's reg property leads to the
smem node being located in different places depending on if the
message ram is being used or not. Let's add a qcom specific
property, qcom,rpm-msg-ram, and point to the device memory from
the SMEM node via a phandle.  As SMEM is a software construct, it
really needs to reside at the root of the DT regardless of whether
it's using the message ram or not.

Cc: Bjorn Andersson <bjorn.andersson@sonymobile.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Andy Gross <agross@codeaurora.org>