feat(shields): add helix shield

PR: #429

app/boards/shields/helix/boards/nice_nano.overlay

@@ -0,0 +1,34 @@
+/*
+ * Copyright (c) 2020 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+&spi1 {
+  compatible = "nordic,nrf-spim";
+  status = "okay";
+  mosi-pin = <6>;
+  // Unused pins, needed for SPI definition, but not used by the ws2812 driver itself.
+  sck-pin = <5>;
+  miso-pin = <7>;
+
+  led_strip: ws2812@0 {
+    compatible = "worldsemi,ws2812-spi";
+    label = "WS2812";
+
+    /* SPI */
+    reg = <0>; /* ignored, but necessary for SPI bindings */
+    spi-max-frequency = <4000000>;
+
+    /* WS2812 */
+    chain-length = <32>; /* number of LEDs */
+    spi-one-frame = <0x70>;
+    spi-zero-frame = <0x40>;
+  };
+};
+
+/ {
+  chosen {
+    zmk,underglow = &led_strip;
+  };
+};