clk: rockchip: rk3568: Include UART clocks in SPL

The clock driver for RK3568 does not include support for UART clocks in
SPL. This result in the following message with high enough loglevel.

  ns16550_serial serial@fe660000: pinctrl_select_state_full: uclass_get_device_by_phandle_id: err=-19

Fix this by including support for UART clocks in SPL.

Fixes: 4a262feb ("rockchip: rk3568: add clock driver")
Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Eugen Hristev <eugen.hristev@collabora.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>