doc: board: apple: Add Apple M1 documentation

Provide preliminary instructions on how to get U-Boot to run on
Apple Silicon Macs.

Signed-off-by: Mark Kettenis <kettenis@openbsd.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/board/apple/index.rst b/doc/board/apple/index.rst
new file mode 100644
index 0000000..8446847
--- /dev/null
+++ b/doc/board/apple/index.rst
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+.. SPDX-License-Identifier: GPL-2.0+
+
+Apple
+=====
+
+.. toctree::
+   :maxdepth: 2
+
+   m1