drm/etnaviv: make THERMAL selectable

drm/etnaviv: make THERMAL selectable

The etnaviv driver causes a link failure if it is built-in but THERMAL
is built as a module:

  drivers/gpu/drm/etnaviv/etnaviv_gpu.o: In function `etnaviv_gpu_bind':
  etnaviv_gpu.c:(.text+0x4c4): undefined reference to `thermal_of_cooling_device_register'
  etnaviv_gpu.c:(.text+0x600): undefined reference to `thermal_cooling_device_unregister'
  drivers/gpu/drm/etnaviv/etnaviv_gpu.o: In function `etnaviv_gpu_unbind':
  etnaviv_gpu.c:(.text+0x2aac): undefined reference to `thermal_cooling_device_unregister'

Adding a Kconfig dependency on THERMAL || !THERMAL to avoid this causes
a dependency loop on x86_64:

  drivers/gpu/drm/tve200/Kconfig:1:error: recursive dependency detected!
  For a resolution refer to Documentation/kbuild/kconfig-language.txt
  subsection "Kconfig recursive dependency limitations"
  drivers/gpu/drm/tve200/Kconfig:1:       symbol DRM_TVE200 depends on CMA
  For a resolution refer to Documentation/kbuild/kconfig-language.txt
  subsection "Kconfig recursive dependency limitations"
  mm/Kconfig:489: symbol CMA is selected by DRM_ETNAVIV
  For a resolution refer to Documentation/kbuild/kconfig-language.txt
  subsection "Kconfig recursive dependency limitations"
  drivers/gpu/drm/etnaviv/Kconfig:2:      symbol DRM_ETNAVIV depends on THERMAL
  For a resolution refer to Documentation/kbuild/kconfig-language.txt
  subsection "Kconfig recursive dependency limitations"
  drivers/thermal/Kconfig:5:      symbol THERMAL is selected by ACPI_VIDEO
  For a resolution refer to Documentation/kbuild/kconfig-language.txt
  subsection "Kconfig recursive dependency limitations"
  drivers/acpi/Kconfig:189:       symbol ACPI_VIDEO is selected by BACKLIGHT_CLASS_DEVICE
  For a resolution refer to Documentation/kbuild/kconfig-language.txt
  subsection "Kconfig recursive dependency limitations"
  drivers/video/backlight/Kconfig:158:    symbol BACKLIGHT_CLASS_DEVICE is selected by DRM_PARADE_PS8622
  For a resolution refer to Documentation/kbuild/kconfig-language.txt
  subsection "Kconfig recursive dependency limitations"
  drivers/gpu/drm/bridge/Kconfig:62:      symbol DRM_PARADE_PS8622 depends on DRM_BRIDGE
  For a resolution refer to Documentation/kbuild/kconfig-language.txt
  subsection "Kconfig recursive dependency limitations"
  drivers/gpu/drm/bridge/Kconfig:1:       symbol DRM_BRIDGE is selected by DRM_TVE200

To work around this, add a new option DRM_ETNAVIV_THERMAL to optionally
enable thermal throttling support and make DRM_ETNAVIV select THERMAL
at the same time.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>