chore: standardize C style headers

Changes made with regex plus some manual tweaks.

Find:
/^\/\*.*\n(?:^\*.*\n)*^.*\*.*?[Cc]opyright.*?(\d{4}) *([0-9A-z,_\- ]+).*?\n(?:^.*\*.*\n)*^.*\*.*?SPDX-License-Identifier:? *(.+) *?\n(?:^.*\*.*\n)*.*\*\//gm

Replace:
/*
 * Copyright (c) $1 $2
 *
 * SPDX-License-Identifier: $3
 */

PR: #565

app/boards/arm/nrf52840_m2/nrf52840_m2.dts

@@ -1,5 +1,6 @@
 /*
  * Copyright (c) 2020 The ZMK Contributors
+ *
  * SPDX-License-Identifier: MIT
  */