xhci: Refactor interrupter code for initial multi interrupter support.

xHC supports several interrupters, each with its own mmio register set,
event ring and MSI/MSI-X vector. Transfers can be assigned different
interrupters when queued. See xhci 4.17 for details.
Current driver only supports one interrupter.

Create a xhci_interrupter structure containing an event ring, pointer to
mmio registers for this interrupter, variables to store registers over s3
suspend, erst, etc. Add functions to create and free an interrupter, and
pass an interrupter pointer to functions that deal with events.

Secondary interrupters are also useful without having an interrupt vector.
One use case is the xHCI audio sideband offloading where a DSP can take
care of specific audio endpoints.

When all transfer events of an offloaded endpoint can be mapped to a
separate interrupter event ring the DSP can poll this ring, and we can mask
these events preventing waking up the CPU.

Only minor functional changes such as clearing some of the interrupter
registers when freeing the interrupter.

Still create only one primary interrupter.

Signed-off-by: Mathias Nyman <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

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