feat(shields): add helix shield

PR: #429

app/boards/shields/helix/helix_left.overlay

@@ -0,0 +1,19 @@
+/*
+ * Copyright (c) 2020 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+ 
+#include "helix.dtsi"
+
+&kscan0 {
+    col-gpios
+        = <&pro_micro_a 3 GPIO_ACTIVE_HIGH>
+        , <&pro_micro_a 2 GPIO_ACTIVE_HIGH>
+        , <&pro_micro_a 1 GPIO_ACTIVE_HIGH>
+        , <&pro_micro_a 0 GPIO_ACTIVE_HIGH>
+        , <&pro_micro_d 15 GPIO_ACTIVE_HIGH>
+        , <&pro_micro_d 14 GPIO_ACTIVE_HIGH>
+        , <&pro_micro_d 16 GPIO_ACTIVE_HIGH>
+        ;
+};