rockchip: rk3399: spl: add UART0 support for SPL

rockchip: rk3399: spl: add UART0 support for SPL

The RK3399-Q7 ("Puma") SoM exposes UART0 as the Qseven UART (i.e. the
serial line available via standardised pins on the edge connector and
available on a RS232 connector).

To support boards (such as the RK3399-Q7) that require UART0 as a
debug console, we match CONFIG_DEBUG_UART_BASE and add the appropriate
iomux setup to the rk3399 SPL code.

As we are already touching this code, we also move the board-specific
UART setup (i.e. iomux setup) into board_debug_uart_init(). This will
be called from the debug UART init when CONFIG_DEBUG_UART_BOARD_INIT
is set.

As the RK3399 needs to use its board_debug_uart_init() function, we
have Kconfig enable it by default for RK3399 builds.

With everything set up to define CONFIG_BAUDRATE via defconfig and
with to have the SPL debug UART either on UART0 or UART2, the configs
for the RK3399 EVB are then update (the change for the RK3399-Q7 is
left for later to not cause issues on applying the change).

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Acked-by: Simon Glass <sjg@chromium.org>