soc: arc: snps_nsim: s/soc.c/smp.c and compile only if CONFIG_SMP=y

The soc.c source only contains code relevant to SMP, and in fact, it is
a file that is left empty when pre-processed from a compiler perspective
if CONFIG_SMP=n. This change renames it to smp.c to make it clear it
contains SMP related code, and, it compiles it conditionally to
CONFIG_SMP=y.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>