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>
diff --git a/configs/sifive_unmatched_defconfig b/configs/sifive_unmatched_defconfig
index 867611b..71c0fb3 100644
--- a/configs/sifive_unmatched_defconfig
+++ b/configs/sifive_unmatched_defconfig
@@ -47,6 +47,7 @@
 CONFIG_CMD_PWM=y
 CONFIG_CMD_GPT_RENAME=y
 CONFIG_CMD_PCI=y
+CONFIG_CMD_POWEROFF=y
 CONFIG_CMD_USB=y
 CONFIG_ENV_SPI_BUS=1
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y