configs: visionfive2: enable SPL_YMODEM_SUPPORT

We can use U-Boot for recovering JH7110 based boards via UART
if CONFIG_SPL_YMODEM_SUPPORT=y.

* Send u-boot-spl.normal.out via XMODEM.
* Send u-boot.itb via YMODEM.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: E. Shattow <lucent@gmail.com>