RISC-V: enable CONFIG_SYSRESET_SBI by default

System reset via the SRST extension in the SBI should be the default.
The driver checks if the extension is available when probing.
So there is no risk in enabling it.

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