clk: rockchip: Export rockchip_clk_register_ddrclk()

This is used by the Rockchip clk driver, export it to allow that
driver to be compiled as a module..

Signed-off-by: Elaine Zhang <zhangqing@rock-chips.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Reviewed-by: Stephen Boyd <sboyd@kernel.org>
Link: https://lore.kernel.org/r/20200914022225.23613-3-zhangqing@rock-chips.com


Signed-off-by: Heiko Stuebner <heiko@sntech.de>