soc: qcom: apr: Don't use reg for domain id

The reg property represents the address and size on the bus that a
device lives, but for APR the parent is a rpmsg bus, which does not have
numerical addresses. Simply defining #address/#size-cells to 1 and 0,
respectively, to silence the compiler is not an appropriate solution.

Replace the use of "reg" with an APR specific property.

Reviewed-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>