The chip contains fractional PLL, however the driver currently hard-codes
one specific PLL setting. Implement generic PLL parameter calculation code,
so any DPI panel with arbitrary pixel clock can be attached to this bridge.

The datasheet for this bridge is not available, the PLL behavior has been
inferred from [1] and [2] and by analyzing the DPI pixel clock with scope.
The PLL limits might be wrong, but at least the calculated values match all
the example code available. This is better than one hard-coded pixel clock
value anyway.

[1] https://github.com/rockchip-linux/kernel/blob/develop-4.19/drivers/gpu/drm/bridge/icn6211.c
[2] https://github.com/tdjastrzebski/ICN6211-ConfiguratorAcked-by: Maxime Ripard <maxime@cerno.tech>
Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Jagan Teki <jagan@amarulasolutions.com>
Cc: Maxime Ripard <maxime@cerno.tech>
Cc: Robert Foss <robert.foss@linaro.org>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
To: dri-devel@lists.freedesktop.org
Signed-off-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220331150509.9838-5-marex@denx.de