rockchip: Fix TPL build without CONFIG_TPL_SERIAL_SUPPORT

If CONFIG_DEBUG_UART is set but CONFIG_TPL_SERIAL_SUPPORT is not, the
serial output should be available in SPL and full U-Boot, but not built
in TPL. However, the rockchip tpl.c instead fails to compile with
undefined references to the debug UART.

Instead, initialise the debug UART and print the TPL banner only if both
CONFIG_DEBUG_UART and CONFIG_TPL_SERIAL_SUPPORT are set.

Signed-off-by:  <chris@arachsys.com>
Reviewed-by: Kever Yang <Kever.yang@rock-chips.com>