configs: sifive: enable poweroff command on Unmatched

Powering off the SiFive HiFive Unmatched board is supported both via the
SBI and GPIO sysreset drivers. See device-tree entry

    compatible = "gpio-poweroff".

Enable the poweroff command.

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