pmdomain: mediatek: Add error messages for missing regmaps

A recent change to the syscon regmap API caused the MediaTek power
controller drivers to fail, as the required regmap could no longer be
retrieved. The error did not have an accompanying message, making the
failure less obvious. The aforementioned change has since been reverted.

Add error messages to all the regmap retrievals, thereby making all
error paths in scpsys_add_one_domain() have visible error messages.

Signed-off-by: Chen-Yu Tsai <wenst@chromium.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20250421090951.395467-1-wenst@chromium.org


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>