genpd: Create a new subsystem directory to host genpd providers

There are currently ~60 users of the genpd provider interface, which
implementations are sprinkled across various subsystems. To simplify with
maintenance let's create a new subsystem (drivers/genpd) and start moving
the providers in there.

My intention is also to host a git tree to collect and to get the patches
tested/integrated through the linux-next tree. Ideally this should release
some of the burden on the soc maintainers.

Note that, I will of course require acks/reviews from the current platform
maintainers, hence the MAINTAINERS file needs to be updated accordingly for
each genpd provider that is moved into the new genpd subsystem.

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