rockchip: Add a way to obtain the main clock device

On Rockchip SoCs we typically have a main clock device that uses the Soc
clock driver. There is also a fixed clock for the oscillator. Add a function
to obtain the core clock.

Signed-off-by: Simon Glass <sjg@chromium.org>

arch/arm/include/asm/arch-rockchip/clock.h

@@ -67,4 +67,6 @@ struct rk3288_grf;
 
 void rk3288_clk_configure_cpu(struct rk3288_cru *cru, struct rk3288_grf *grf);
 
+int rockchip_get_clk(struct udevice **devp);
+
 #endif