doc: Add device-tree-bindings for atsha204 and atsha204a

Document trivial bindings for atsha204 and atsha204a.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Heiko Schocher <hs@denx.de>
diff --git a/doc/device-tree-bindings/misc/atsha204.txt b/doc/device-tree-bindings/misc/atsha204.txt
new file mode 100644
index 0000000..3adc121
--- /dev/null
+++ b/doc/device-tree-bindings/misc/atsha204.txt
@@ -0,0 +1,4 @@
+Atmel ATSHA204 and ATSHA204A i2c h/w symmetric crypto module
+
+Required properties:
+- compatible : should be "atmel,atsha204" or "atmel,atsha204a"