msm: rtb: Only select separate cpus if SMP

The register trace buffer has an option to log entries from CPUs
separately. This only make sense on an SMP system and the parsing
tools make certain assumptions if the step size is 1, which is
generally equal to the number of cpus when MSM_RTB_SEPARATE_CPUS
is selected. Avoid problems by requiring CONFIG_SMP to be selected
when trying to use MSM_RTB_SEPARATE_CPUS.

Change-Id: I9ee6cae7b4136e9b7ec1e60a4e4e9722e8b0ca8c
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>