soc: qcom: smp2p: Fix makefile dependency

SMP2P compilation depends on the loopback server client. Move this
file under the correct defconfig for smp2p to be independent of the
unit tests.

CRs-Fixed: 2120485
Change-Id: I8600f920b34ba474fe3763a0cccda79b9adcea95
Signed-off-by: Chris Lew <clew@codeaurora.org>