riscv: enable CONFIG_DEBUG_UART by default

Most boards don't enable the pre-console buffer. So we will not see any
early messages. OpenSBI 1.3 provides us with the debug console extension
that can fill this gap.

For S-Mode U-Boot enable CONFIG_DEBUG_UART by default.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>