drm/etnaviv: add support for slave interface clock

The slave interface clock is a clock input found on newer cores to gate
the register interface. For now we simply ungate it when the GPU is in
active state.

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

drivers/gpu/drm/etnaviv/etnaviv_gpu.h

@@ -147,6 +147,7 @@ struct etnaviv_gpu {
 
 	/* Power Control: */
 	struct clk *clk_bus;
+	struct clk *clk_reg;
 	struct clk *clk_core;
 	struct clk *clk_shader;