It seems reasonable to collect the core parts for the generic PM domain,
along with its corresponding provider drivers. Therefore let's move the
files from drivers/base/power/ to drivers/pmdomain/ and while at it, let's
also rename the files accordingly.

Moreover, let's also update MAINTAINERS to reflect the update.

Cc: Kevin Hilman <khilman@kernel.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Rafael J. Wysocki <rafael@kernel.org>
Link: https://lore.kernel.org/r/20231213113305.29098-1-ulf.hansson@linaro.org