rockchip: clk: rk3399: add clk_enable function and support USB HOST0/1

The generic ehci-driver (ehci-generic.c) will try to enable the clocks
listed in the DTSI. If this fails (e.g. due to clk_enable not being
implemented in a driver and -ENOSYS being returned by the clk-uclass),
the driver will bail our and print an error message.

This implements a minimal clk_enable for the RK3399 and supports the
clocks mandatory for the EHCI controllers; as these are enabled by
default we simply return success.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>