PM: domains: Move genpd and its governor to the pmdomain subsystem

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