soc: apple: Move power-domain driver to the genpd dir

To simplify with maintenance let's move the apple power-domain driver to
the new genpd directory. Going forward, patches are intended to be managed
through a separate git tree, according to MAINTAINERS.

Cc: Hector Martin <marcan@marcan.st>
Cc: Sven Peter <sven@svenpeter.dev>
Cc: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Cc: <asahi@lists.linux.dev>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>