rockchip: rk3399: Add option to print on UART3.

rockchip: rk3399: Add option to print on UART3.

The RK3399 SPL does not use a pinctrl driver to setup the UART pins.
Instead it works based on config macros, which set the base address
of the actual UART block.

Currently the RK3399 SPL support UART0 and UART2.
This patch adds UART3 in the same way as UART0.

Signed-off-by: Christoph Muellner <christoph.muellner@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>