efi_selftest: enable CONFIG_CMD_POWEROFF

For automating testing we should be able to power off the test system.
The implementation of EFI_RESET_SHUTDOWN requires the do_poweroff()
function which is only available if CONFIG_CMD_POWEROFF=y.

Enable CONFIG_CMD_POWEROFF if PSCI reset is available.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>