drm/etnaviv: implement cooling support for new GPU cores

GPU cores with the DYNAMIC_FREQUENCY_SCALING feature bit set expect the
platform to provide the clock scaling and ignore any requests to use the
internal FSCALE divider. Writes to this register still work, but don't
have any effect on the GPU clock frequency.

Save the initial core and shader clock frequency and ask the platform
to provide a slower clock when cooling is requested.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>