dt: Add additional option bindings for IDT VersaClock

The VersaClock driver now supports some additional bindings to support
child nodes which can configure optional settings like mode, voltage
and slew.

This patch updates the binding document to describe what is available
in the driver.

Signed-off-by: Adam Ford <aford173@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200603154329.31579-2-aford173@gmail.com
Signed-off-by: Stephen Boyd <sboyd@kernel.org>

include/dt-bindings/clk/versaclock.h

@@ -0,0 +1,13 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+
+/* This file defines field values used by the versaclock 6 family
+ * for defining output type
+ */
+
+#define VC5_LVPECL	0
+#define VC5_CMOS	1
+#define VC5_HCSL33	2
+#define VC5_LVDS	3
+#define VC5_CMOS2	4
+#define VC5_CMOSD	5
+#define VC5_HCSL25	6