soc: qcom: smem: Add qcom_smem_is_available()

Avoid having to look up a dummy item from SMEM to detect if it is
already available or if we need to defer probing.

Reviewed-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
Link: https://lore.kernel.org/r/20230531-rpm-rproc-v3-7-a07dcdefd918@gerhold.net


Signed-off-by: Bjorn Andersson <andersson@kernel.org>