sandbox: Exit when SYSRESET_POWER_OFF is requested

At present this returns but it seems better to just exit sandbox
immediately.

Signed-off-by: Simon Glass <sjg@chromium.org>

reset
diff --git a/arch/sandbox/cpu/state.c b/arch/sandbox/cpu/state.c
index d3b9c05..2333240 100644
--- a/arch/sandbox/cpu/state.c
+++ b/arch/sandbox/cpu/state.c
@@ -356,6 +356,7 @@
 	/* No reset yet, so mark it as such. Always allow power reset */
 	state->last_sysreset = SYSRESET_COUNT;
 	state->sysreset_allowed[SYSRESET_POWER] = true;
+	state->sysreset_allowed[SYSRESET_POWER_OFF] = true;
 
 	memset(&state->wdt, '\0', sizeof(state->wdt));
 	memset(state->spi, '\0', sizeof(state->spi));