Support reading Subsystem ID from Device Tree

Support reading Subsystem ID from Device Tree

Merge series from Stefan Binding <sbinding@opensource.cirrus.com>:

In PC systems using ACPI, the driver is able to read back an SSID from
the _SUB property. This SSID uniquely identifies the system, which
enables the driver to read the correct firmware and tuning for that
system from linux-firmware. Currently there is no way of reading this
property from device tree. Add an equivalent property in device tree
to perform the same role.