drm/bridge: tc358767: Support write-only registers

Most registers are read-writable, but some are only RO or even WO.
regmap does not support using readable_reg and wr_table when outputting
in debugfs, so switch to writeable_reg.
First check for RO or WO registers and fallback tc_readable_reg() for the
leftover RW registers.

Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Reviewed-by: Robert Foss <rfoss@kernel.org>
Link: https://lore.kernel.org/r/20240904120546.1845856-4-alexander.stein@ew.tq-group.com


Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240904120546.1845856-4-alexander.stein@ew.tq-group.com