rockchip: px30: support debug uart on UART0

UART0 can obviously also be used for debug uart in U-Boot, so let's add
its support.

Cc: Quentin Schulz <foss+uboot@0leil.net>
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>