soc: qcom: smsm: Add missing mailbox dependency to Kconfig

soc: qcom: smsm: Add missing mailbox dependency to Kconfig

Since the smsm driver got the ability to interact with the mailbox using
the mailbox subsystem and not just syscon, we need to add the dependency
to kconfig as well to avoid compile errors.

Fixes: 75287992f58a ("soc: qcom: smsm: Support using mailbox interface")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202406180006.Z397C67h-lkp@intel.com/
Signed-off-by: Luca Weiss <luca@lucaweiss.eu>
Reviewed-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Link: https://lore.kernel.org/r/20240628-smsm-kconfig-v1-1-117d5af4ba1f@lucaweiss.eu
Signed-off-by: Bjorn Andersson <andersson@kernel.org>