drm/rcar-du: dsi: Clean up CLOCKSET1 CLKINSEL macros

drm/rcar-du: dsi: Clean up CLOCKSET1 CLKINSEL macros

Introduce CLOCKSET1_CLKINSEL_MASK macro and remove bitshift from
values to make this bitfield usable with FIELD_PREP(). There are
no users of this bitfield, hence no updates to the DSI driver.

Do not convert bits and bitfields to BIT() and GENMASK() yet, to be
consisten with the current style. Conversion to BIT() and GENMASK()
macros is done at the very end of this series in the last two patches.

Reviewed-by: Tomi Valkeinen <tomi.valkeinen+renesas@ideasonboard.com>
Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Link: https://patch.msgid.link/20251028232959.109936-7-marek.vasut+renesas@mailbox.org
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>