drm/rcar-du: dsi: Clean up TXVMPSPHSETR DT macros

drm/rcar-du: dsi: Clean up TXVMPSPHSETR DT macros

Introduce TXVMPSPHSETR_DT_MASK macro and use FIELD_PREP() to generate
appropriate bitfield from mask and value without bitshift.

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/20251028232959.109936-8-marek.vasut+renesas@mailbox.org
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>