PM / Domains: Introduce a genpd OF helper that removes a subdomain

We already have the of_genpd_add_subdomain() helper, but no corresponding
of_genpd_remove_subdomain(), so let's add it. Subsequent changes starts to
make use of it.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Rafael J. Wysocki <rafael@kernel.org>