rockchip: Use rockchip_get_clk() to obtain the SoC clock

The current code picks the first available clock. In U-Boot proper this is
the oscillator device, not the SoC clock device. As a result the HDMI display
does not work.

Fix this by calling rockchip_get_clk() instead.

Fixes: 135aa950 (clk: convert API to match reset/mailbox style)
Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Anatolij Gustschin <agust@denx.de>