clk: rockchip: remove redundant checking of device_node

rockchip_clk_of_add_provider is used by sub-clk driver which
already call of_iomap before calling it. If device_node does
not exist, of_iomap returns NULL which will fail to init the
sub-clk driver. So really it's redundant.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>