drm/etnaviv: add etnaviv cooling device

Each Vivante GPU contains a clock divider which can divide the GPU clock
by 2^n, which can lower the power dissipation from the GPU.  It has been
suggested that the GC600 on Dove is responsible for 20-30% of the power
dissipation from the SoC, so lowering the GPU clock rate provides a way
to throttle the power dissiptation, and reduce the temperature when the
SoC gets hot.

This patch hooks the Etnaviv driver into the kernel's thermal management
to allow the GPUs to be throttled when necessary, allowing a reduction in
GPU clock rate from /1 to /64 in power of 2 steps.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>