dt-bindings: Add a clocks property to the simple-framebuffer binding

A simple-framebuffer node represents a framebuffer setup by the firmware /
bootloader. Such a framebuffer may have a number of clocks in use, add a
property to communicate this to the OS.
Signed-off-by: NHans de Goede <hdegoede@redhat.com>
Reviewed-by: NMike Turquette <mturquette@linaro.org>
Acked-by: NGeert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: NMaxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: NGrant Likely <grant.likely@linaro.org>
Signed-off-by: NTomi Valkeinen <tomi.valkeinen@ti.com>

Documentation/devicetree/bindings/video/simple-framebuffer.txt

 Simple Framebuffer
 
-A simple frame-buffer describes a raw memory region that may be rendered to,
-with the assumption that the display hardware has already been set up to scan
-out from that buffer.
+A simple frame-buffer describes a frame-buffer setup by firmware or
+the bootloader, with the assumption that the display hardware has already
+been set up to scan out from the memory pointed to by the reg property.
 
 Required properties:
 - compatible: "simple-framebuffer"
@@ -14,6 +14,12 @@ Required properties:
   - r5g6b5 (16-bit pixels, d[15:11]=r, d[10:5]=g, d[4:0]=b).
   - a8b8g8r8 (32-bit pixels, d[31:24]=a, d[23:16]=b, d[15:8]=g, d[7:0]=r).
 
+Optional properties:
+- clocks : List of clocks used by the framebuffer. Clocks listed here
+           are expected to already be configured correctly. The OS must
+           ensure these clocks are not modified or disabled while the
+           simple framebuffer remains active.
+
 Example:
 
 	framebuffer {