dts: bindings: usb: Add enum to maximum-speed property

Add enum to list what values of maximum-speed are exceptable since
enum's get represented as strings in DT.  This also allows us to
generate a code enum to correspond to the string.

We also introduce include/dt-bindings/usb/usb.h which is a hand coded
definition of the enum.  We don't have a great way to generate this
right now, however it would be better if we did.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>

dts/bindings/usb/usb.yaml

@@ -26,6 +26,11 @@ properties:
                    via DT, USB controllers should use their maximum
                    hardware capability.
       generation: define
+      enum:
+         - "low-speed"
+         - "full-speed"
+         - "high-speed"
+         - "super-speed"
 
     label:
       type: string

include/dt-bindings/usb/usb.h

@@ -0,0 +1,22 @@
+/*
+ * Copyright (c) 2019 Linaro Limited
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+#ifndef ZEPHYR_INCLUDE_DT_BINDINGS_USB_USB_H_
+#define ZEPHYR_INCLUDE_DT_BINDINGS_USB_USB_H_
+
+/* Ideally we'd generate this enum to match what's coming out of the YAML,
+ * however, we dont have a good way to know how to name such an enum from
+ * the generation point of view, so for now we just hand code the enum.  This
+ * enum is expected to match the order in the yaml (dts/bindings/usb/usb.yaml)
+ */
+
+enum dt_usb_maximum_speed {
+	DT_USB_MAXIMUM_SPEED_LOW_SPEED,
+	DT_USB_MAXIMUM_SPEED_FULL_SPEED,
+	DT_USB_MAXIMUM_SPEED_HIGH_SPEED,
+	DT_USB_MAXIMUM_SPEED_SUPER_SPEED,
+};
+
+#endif /* ZEPHYR_INCLUDE_DT_BINDINGS_USB_USB_H_ */