drm/imagination: Implement power management

Add power management to the driver, using runtime pm. The power off
sequence depends on firmware commands which are not implemented in this
patch.

Changes since v8:
- Corrected license identifiers

Changes since v5:
- Use RUNTIME_PM_OPS() to declare PM callbacks
- Add Kconfig dependency on CONFIG_PM

Changes since v4:
- Suspend runtime PM before unplugging device on rmmod

Changes since v3:
- Don't power device when calling pvr_device_gpu_fini()
- Documentation for pvr_dev->lost has been improved
- pvr_power_init() renamed to pvr_watchdog_init()
- Use drm_dev_{enter,exit}

Changes since v2:
- Use runtime PM
- Implement watchdog

Signed-off-by: Sarah Walker <sarah.walker@imgtec.com>
Signed-off-by: Donald Robson <donald.robson@imgtec.com>
Reviewed-by: Maxime Ripard <mripard@kernel.org>