rockchip: rk3399: spl: ensure that debug_uart_init is called

With the latest changes to add support for the Chromebook Bob,
initialisation through debug_uart_init() did no longer get called for
other targets.

Fix this, by moving debug_uart_init() out of the Bob-specific

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>