bindings: spmi: spmi-pmic-arb: Add description for "qcom,mid"

Add a description for the "qcom,mid" property which corresponds
to the SPMI master ID of the SPMI PMIC arbiter controller. It is
most useful in systems with multiple masters, e.g. when there are
multiple SoCs connected to a single SPMI bus.

Change-Id: I376e6f569f9bbea44ba3930480a330fb1dd2c2de