sandbox: cros_ec: Support EC_CMD_ENTERING_MODE emulation

Emualate this function which is used with Chrome OS verified boot.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/misc/cros_ec_sandbox.c b/drivers/misc/cros_ec_sandbox.c
index df41e82..7509612 100644
--- a/drivers/misc/cros_ec_sandbox.c
+++ b/drivers/misc/cros_ec_sandbox.c
@@ -459,6 +459,8 @@
 	case EC_CMD_MKBP_STATE:
 		len = cros_ec_keyscan(ec, resp_data);
 		break;
+	case EC_CMD_ENTERING_MODE:
+		break;
 	default:
 		printf("   ** Unknown EC command %#02x\n", req_hdr->command);
 		return -1;