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: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Mike Turquette <mturquette@linaro.org>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Grant Likely <grant.likely@linaro.org>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>