drm: bridge: icn6211: Convert to regmap

To make debugging easier, convert driver to regmap. Implement read and write
regmap tables for known registers, keep all known register readable and mark
those which are obviously read-only as not writeable.

Use common I2C regmap for the I2C configuration, implement custom regmap bus
for DSI configuration. The later is mandatory as this chip requires one extra
byte set to read access length between register address and data.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Jagan Teki <jagan@amarulasolutions.com>
Cc: Mark Brown <broonie@kernel.org>
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
Acked-by: Mark Brown <broonie@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220403171004.368464-1-marex@denx.de