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>
diff --git a/arch/arm/mach-rockchip/rk3288/Makefile b/arch/arm/mach-rockchip/rk3288/Makefile
index 6f62375..82b00a1 100644
--- a/arch/arm/mach-rockchip/rk3288/Makefile
+++ b/arch/arm/mach-rockchip/rk3288/Makefile
@@ -4,6 +4,7 @@
 # SPDX-License-Identifier:      GPL-2.0+
 #
 
+obj-y += clk_rk3288.o
 obj-y += reset_rk3288.o
 obj-y += sdram_rk3288.o
 obj-y += syscon_rk3288.o