dt-bindings: display: bridge: icn6211: Add support for external REFCLK

The ICN6211 is capable of deriving its internal PLL clock from either
MIPI DSI HS clock, external REFCLK clock, or even internal oscillator.
Currently supported is only the first option. Document support for
external REFCLK clock input in addition to that.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Jagan Teki <jagan@amarulasolutions.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Rob Herring <robh@kernel.org>
Cc: Robert Foss <robert.foss@linaro.org>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: dri-devel@lists.freedesktop.org
Cc: devicetree@vger.kernel.org
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220801131747.183041-1-marex@denx.de