riscv: qemu: Enable PRE_CONSOLE_BUFFER

By default the video console only outputs messages after it's ready.
Messages before that won't show on the video console, but U-Boot has
an option to buffer the console messages before it's ready.

Enable this support, and carefully select an address for the buffer.

Signed-off-by: Bin Meng <bmeng@tinylab.org>
Reviewed-by: Rick Chen <rick@andestech.com>
Reviewed-by: Simon Glass <sjg@chromium.org>