The per-device PM QoS locking requires a spinlock to be used. The reasons
are:
 - an alignement with the PM QoS core code, which is used by the per-device
   PM QoS code for the constraints lists management. The PM QoS core code
   uses spinlocks to protect the constraints lists,
 - some drivers need to use the per-device PM QoS functionality from
   interrupt context or spinlock protected context.
   An example of such a driver is the OMAP HSI (high-speed synchronous serial
   interface) driver which needs to control the IP block idle state
   depending on the FIFO empty state, from interrupt context.
Reported-by: NDjamil Elaidi <d-elaidi@ti.com>
Signed-off-by: NJean Pihet <j-pihet@ti.com>
Signed-off-by: NRafael J. Wysocki <rjw@sisk.pl>