SDCA System Suspend Support

SDCA System Suspend Support

Merge series from Charles Keepax <ckeepax@opensource.cirrus.com>:

Add support for system suspend into the class driver, now split
out into a separate patch series.

Where we got to on the previous discussion, was we don't currently
have any parts requiring download on runtime resume, doing so
will add noticeable delay to the runtime resume, and we are not
blocking someone from adding support for firmware download on
runtime resume in the future. Also as runtime resume is really
a kernel concept and power rails are primarily controlled by
ACPI it is quite unlikely anyone will actually power down the
part on a runtime suspend anyway. So this version of the chain
still only downloads firmware on probe and system resume.