clk: ccf: Add missing #include <dm/uclass.h> to clk-mux.c

After adding custom get_rate helper function it was necessary to include
<dm/uclass.h> to avoid warnings about missing uclass_get_device_by_name.

Signed-off-by: Lukasz Majewski <lukma@denx.de>
Series-to: u-boot
1 file changed