Driver parameters:

- firmware_command_timeout, int: a number of seconds that the host waits for
  the DSP to respond to a synchronization request or a command. Can be changed
  at runtime through the following sysfs entry:
    /sys/module/xrp/parameters/firmware_command_timeout

- firmware_reboot, 0/1: controls whether the driver reboots firmware on
  command timeout. Enabled by default and can be changed at runtime through
  the following sysfs entry: /sys/module/xrp/parameters/firmware_reboot

- loopback, 0/1/2/3: controls level of interaction between the driver and
  the firmware.
  0: normal operation. The driver loads firmware, controls DSP and interacts
     with the firmware through shared memory;
  1: no-communication loopback. The driver loads the firmware and controls
     DSP, but does not communicate with firmware. Initial synchronization
     is not performed and command submission is completed immediately after
     all normal preparation steps related to buffer mapping. Communication
     area is not touched by the driver.
  2: no-control loopback. The driver loads the firmware, but does not control
     DSP nor does it communicate with the firmware. Neither communication
     area nor DSP MMIO area are touched by the driver.
  3: no-firmware loopback. The driver doesn't load firmware, doesn't control
     DSP and doesn't communicate with DSP.
