drm: 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. Add support for external
REFCLK clock input in addition to that.

There is little difference between these options, except that in case
of MIPI DSI HS clock input, the HS clock are pre-divided by a fixed /4
divider before being fed to the PLL input, while in case of external
REFCLK, the RECLK clock are fed directly into the PLL input.

Per exceptionally poor documentation, the REFCLK must be in range of
10..154 MHz.

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: Robert Foss <robert.foss@linaro.org>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220801131747.183041-2-marex@denx.de