PM / Domains: Allow overriding the ->xlate() callback

Allow generic power domain providers to override the ->xlate() callback
in case the default genpd_xlate_onecell() translation callback is not
good enough.

One potential use-case for this is to allow generic power domains to be
specified by an ID rather than an index.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Acked-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>