power: reset: syscon-poweroff: get regmap from parent node

power: reset: syscon-poweroff: get regmap from parent node

Just like syscon-reboot device, the syscon-poweroff is supposed to be a
child of syscon node, thus we can take the same approach as
syscon-poweroff: deprecate the 'regmap' field in favor of taking it from
the parent's node.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230901120057.47018-3-krzysztof.kozlowski@linaro.org
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>