soc: tegra: Move powergate-bpmp driver to the genpd dir

To simplify with maintenance let's move the powergate-bpmp driver to the
new genpd directory. Going forward, patches are intended to be managed
through a separate git tree, according to MAINTAINERS.

Note that, we leave the pmc driver in the soc directory for now, as it
looks like it may need some re-structuring before it's ready to be moved.

Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: Jonathan Hunter <jonathanh@nvidia.com>
Cc: Mikko Perttunen <mperttunen@nvidia.com>
Cc: <linux-tegra@vger.kernel.org>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>