drm/rcar-du: dsi: Document PHTR TESTDOUT as bitfield

drm/rcar-du: dsi: Document PHTR TESTDOUT as bitfield

The register PHTR bitfield TESTDOUT is not a single bit, but a
bitfield. Add a MASK macro and a _TEST bitfield value, which is
used by the driver to poll for completion. No functional change.

Reviewed-by: Tomi Valkeinen <tomi.valkeinen+renesas@ideasonboard.com>
Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Link: https://patch.msgid.link/20251028232959.109936-4-marek.vasut+renesas@mailbox.org
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>